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The Vault - Fallout Wiki
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For this location in Fallout 2, see Military base ruins.
Mariposa Military Base
Fo1 Mariposa .png
Map MarkerMilitary Base
FactionsUnited States Army (pre-War)
The Unity (2102-2162)
Enclave (2236-2237)
Second generation super mutants (2241)
LeadersColonel Robert Spindel (2076-2077)
Captain Roger Maxson (October 2077)
Lieutenant (2161)
Colonel Sanders (2236-2237)
Melchior (2241)
QuestsDestroy the source of the Mutants.
TerminalsVats Control Computer
Gametitle-FO1.pngGametitle-FO2.pngGametitle-FO3.pngGametitle-FNV.pngGametitle-FNV DM.pngGametitle-FB.png
Gametitle-FO1.pngGametitle-FO2.pngGametitle-FO3.pngGametitle-FNV.pngGametitle-FNV DM.pngGametitle-FB.png

The Mariposa Military Base was a top-secret United States military facility located somewhere in the area of the Mariposa unincorporated community in California. It was built to house Forced Evolutionary Virus research previously conducted at the West Tek Research Facility.

The Mariposa Military Base can be found thirteen squares west of Vault 13.


The FEV Project

Layout overview.

In 2076, the NBC division of West-Tek achieved breakthrough results in the Pan-Immunity Virion Project. The United States Defense Department, in fear of international espionage, moved a military team under the command of Colonel Robert Spindel and Captain Roger Maxson onto the site to secure and oversee the project, now dubbed the FEV (Forced Evolutionary Virus) project.[1] On January 7, 2077, all Californian FEV research was moved to the newly constructed Mariposa Military Base from the West Tek facility to commence testing of the virus on human subjects.[2][3] The security team was transferred to the newly constructed base as well, to provide protection for the research going on within the facility. They were not informed of the nature of the research.[4]

This secrecy backfired shortly before October 10, 2077, when soldiers stationed at the base discovered the human testing that was going on at the base, made worse by the fact that the scientists were using military prisoners as test subjects. Colonel Spindel suffered a nervous breakdown, locking himself in his office, leaving Captain Maxson to handle the deteriorating situation at the base. The soldiers were screaming for blood and became increasingly aggressive.[4] When two days later, on October 12, the Captain stopped a soldier from murdering a scientist, he ordered the scientists to be interrogated.[5]

The first scientist to be interrogated by Captain Maxson was Robert Anderson, Chief Researcher of the research team at Mariposa. Anderson explained that the testing at the facility was sanctioned and, in fact, ordered by the government, as he outlined the experiments to the Captain. When Maxson refused to believe him, the scientist lost his nerve and started screaming how he was just following orders and that he was also a military man. In response, Captain Maxson murdered him.[6]

This established Maxson as the leader of the rebellion. His position was further reinforced on October 15, when Colonel Spindel committed suicide as his men broke into his office.[7] Subsequent scientist interrogations invariably ended in executions. Erin Shellman held out the longest, almost convincing the Captain that the experiments were really ordered by the government.[8] On October 20, 2077, Captain Maxson declared his unit in full secession from the United States, attempting to force the government to respond to the situation at Mariposa. No response came. Two days later, he ordered the families of soldiers under his command to take shelter within the facility.[9]

The killing of Robert Anderson effectively established Maxson as the leader of the rebellion. His position was further reinforced just two days later on October 15, when he attempted to speak to Colonel Spindel through the door of his office. It soon became clear that the Colonel had lost touch with reality, so Maxon and several of his men broke down the door just in time to hear the Colonel apologize and shoot himself. Subsequent scientist interrogations invariably ended in executions. Erin Shellman held out the longest by October 18, finally convincing the Captain that the experiments were really ordered by the government with her detailed account.[10] On October 20, 2077, Captain Maxson declared his unit in full secession from the United States over the radio, attempting to force the government to respond to the situation at Mariposa. No response came. A day later, he ordered the families of soldiers under his command to take shelter within the facility.[10]

On October 23, 2077, the Great War struck. As Maxson was halfway through prying the story from Head Researcher Leon von Felden, the facility lost contact with the outside world as nuclear weapons started to drop. Spared the nuclear devastation, Mariposa protected the inhabitants from nuclear fallout flooding the wasteland. Fearing that China would soon make up for the oversight, on October 24, Maxson ordered his soldiers and their families to prepare to vacate the base the next day.[11]

On October 25, Sgt. Platner volunteered to take atmospheric readings outside the base. Reporting no significant amounts of radiation in the atmosphere, final preparations for the exodus were undertaken. On October 26, Maxson ordered the remains of the scientists to be buried in the wastes outside the base. A day later, on October 27, former US servicemen and their families left the base under the lead of Captain Roger Maxson, heading for the Lost Hills government bunker in the south.[12][13]

The Rise of the Master

FEV vats at Mariposa

The base was left sealed, but over the years, it slowly fell into disrepair, and eventually various creatures made their way inside, mutating due to F.E.V. exposure and infesting the surrounding area, attacking fledgling human settlements and caravans. The problem was becoming increasingly severe, to the point when Harold, a caravan master from the Hub, mounted an expedition in 2102 to locate and destroy the source of mutants.[14]

Dr. Richard Grey was one of the explorers who was part of the expedition. Harold led the men into Mariposa, but failed to account for the automated defenses, which were still online, and promptly attacked the explorers. Taking heavy casualties, they made their way to the deepest, fourth level, the F.E.V vats, where a crane smashed into the two surviving explorers, Harold and Richard Grey. The former was knocked out, the latter landed in one of the virus vats, succumbing to a severe overdose.[15]

In pain, horribly mutated, he crawled into the control room, as his intellect grew. In a few days, he was neurolinked to the base computer, drawing upon its sizable data banks.[16] As he created the idea of the Unity, wanderers began to enter the facility, and Grey patiently captured and dipped them, in order to create beings as perfect as him. When the first true super mutants were created, he set out to create a true mutant army, at the same time deciding to look for a permanent base of operations. In 2155, one of his patrols located a group of vault dwellers from the Los Angeles Vault, and he made the decision to transfer himself there, occupying the vault.[17]

The base was entrusted to the Lieutenant, one of the greatest super mutant minds to exist. Mariposa was rebuilt and cleaned up, once again becoming the magnificent military base it was before the war. Super mutants roamed the halls, restocking the armories and medical bays and soon it became the staging area for their assault on humanity, as well as the only super mutant production center.[18]

Then came the Vault Dweller in 2162. It is unknown whether or not he infiltrated the base or blasted his way in – suffice to say, once he was through, Mariposa was little more than a caved-in, smoldering ruin.

Enclave's excavations

The existence of the Mariposa Military Base was listed in Enclave records, which enabled the Enclave to find the base and begin their excavations.[19] On July 20th, 2236, Enclave scouts (re)discovered the ruins of the Mariposa Military Base.[20] The Chemical Corps and other scientists began to scour the base, while assault squads began gathering slaves to use for excavating the base.[21] In the process, they capture Melchior from the town of Redding.[22] Excavations began in September. The slaves and construction crews dug into the collapsed base, removing tons of rock. Unprotected wastelanders were exposed to low levels of FEV in the ruins, released after the base self destructed and mutations began to occur. At the same time, Frank Horrigan is exposed to the virus and sent to Control station Enclave for study.[23] By October, Melchior (now mutating) realized that the Enclave would execute him and the others as soon as they recover the FEV and virus data. He began secretly stashing away weapons for the mutants in anticipation of the confrontation.[24] The excavations are complete in January 2237. Shortly after Enclave scientists manage to obtain a pure sample of FEV and leave the base, the mutants rose up against their Enclave overseers. After suffering heavy casualties, the Enclave forces fell back and sealed the mutants in.[25] The remaining first and second generation super mutant slaves remained in the Base, and the group formed a new community under the leadership of Melchior, whose magic talents had allowed them to hide the weapons.



Fo1 Mariposa Surface.png

Mariposa was a reinforced military base, constructed in the middle of nowhere, in the side of a mountain. The exterior had a characteristic white lining and massive air conditioning vent providing fresh air for the complex. Inside, it consisted of four levels, stretching across a mile of depth.

Stronghold Level 1

Fo1 Mariposa Stronghold 1.png

Square footage: 30000-40000 ft

The first level of Mariposa housed the gear needed for any excursions, as well as a staging area for patrols. Oddly, the main control computers for the base's force fields and robotic guardians were located here, allowing any infiltrator to considerably sabotage the base, just by altering the Pest Control setting for the robobrains, in addition to altering the force field status remotely. Apart from that, guard barracks were located on this level, for off-duty surface security teams.

Stronghold Level 2

Fo1 Mariposa Stronghold 2.png

Square footage: 60000 ft

The second level of the stronghold is one of the largest, and for good reason. It houses the primary barracks, with an adjoining medical bay and armory. Both are well stocked with medical supplies and ordnance respectively. The medical bay is particularly well equipped, as it includes four beds with integrated medical monitoring, an operating theater, and full hygienic facilities.

The barracks hold a lot of super mutants of varying degrees of strength. Both the armory and the medbay offer plenty of loot, though in the latter case, the player has to negotiate a super mutant with a particularly small cranium. Lead works well. However, visiting the second level is not in any way necessary to complete the game.

Loot on level 2 of Mariposa

Stronghold Level 3

Fo1 Mariposa Vats 1.png

Square footage: 80000 ft

The third level was divided in two: one was the general access area, where the assembly room and elevator to the commander's office were located; the second a military prison with an elevator going directly to the Vats. When Mariposa was operating under government jurisdiction, experimenting with FEV, only scientists and their security staff were allowed into the prison area and the Vats - soldiers under Colonel Spindel and Roger Maxson were (rightly so) barred from entry.


Fo1 Mariposa Vats 2.png

Square footage: 60000 ft

Arguably the heart of the Mariposa Military Base, the deepest level of the installation is built around a central, ventilated chamber housing the vats filled with the Forced Evolutionary Virus alongside a nutrient solution. There are three massive vats, with a catwalk suspended overhead. The large airshaft vents the mutagenic fumes outside through the vent above the entrance. It also doubles as an attachment point for the manipulator arm and crane used to place subjects within the vats for mutation and retrieve them. The Master relied on a crude dipping procedure to create supermutants: Prisoners placed in protective bondage are first rolled into the central area of the catwalk and then an operator uses the crane to lift them up, suspend over the Vat, and then release the subject. The subsequent instant total immersion ensures rapid changes begin to manifest almost immediately.[26] Failed conversions are typically discarded and those that survive will sometimes lurk around the base of the Vats.

The control room overlooks the Vats. It contains the control consoles, used to monitor subjects, as well as the central access console for the Mariposa mainframe, which allows a crafty hacker to access personnel logs and even set the base to self-destruct. Attaching an explosive to the terminal will also force the self destruct, except it will also cause alarms to ring out throughout the base.


Mariposa Encryption Codes
17652-1028-1285362 - 1 minute silent self destruct sequence
67209-5574-7805564 - 30 second self destruct, set base to alert
55658-8678-1251533 - set base to alert status
31914-1041-1251514 - 3 minute silent self destruct sequence
53478-2565-7763929 - 3 minute self destruct sequence, set base to alert

Together with the scientists' quarters (used around the Master's time to house the supermutant containment squad, Vat Team 9), it forms the top secret portion of the facility, accessible from the cell block block and kept off limits to soldiers before the war. The other, general access section of the level housed the central robot maintenance facility, as well as the base commander's quarters. Once used by Colonel Spindel, the spartan office was claimed by the Lieutenant during the Master's rise.

Related quests

Behind the scenes

  • According to Chris Taylor, Mariposa Military Base is based on Fort Ord, an old military base that has been shut down near Monterey Bay. It was called Base Omega in early design documents.
  • The name of the base, Mariposa, in Spanish means butterfly. This is a probable reference to the metamorphosis that occurs in a caterpillar turning it into a "better" creature, in this case a butterfly. When pure humans are dipped into the FEV vats they transform into a "better" creature, in this case, a smarter/faster/stronger super mutant.



  • In a few cases the yellow force fields' device cannot be deactivated by the use of a tool. If that happens it's only necessary to simply use the Repair skill directly into the force field device.

See also

Related holodisks


The Mariposa Military Base appears in Fallout and Fallout 2. It is mentioned in Fallout 3, a Fallout: New Vegas loading screen, and the Fallout: New Vegas add-on Dead Money.


  1. The Vault Dweller: "{207}{}{Base Information}"
    Power management and mainframe terminal: "{211}{}{West Tech Research Facility: Founded in 2002 as a private contractor for the United States government, the company initially consisted of two divisions--the Advanced Weapons Research and the Biomedical Sciences divisions.} {241}{}{ In 2069, West Tech was the single largest contractor for the United States government; its largest contract being Powered Infantry Armor Model T-51b. }"
    "{242}{}{In light of significant advances in 2076 by the NBC on the Pan-Immunity Virion Project, the United States Defense Department, in fear of international espionage,}"
    "{243}{}{ moved a team onto the site to secure and oversee the project, now dubbed the FEV (Forced Evolutionary Virus) project.}"
  2. The Vault Dweller: "{209}{}{Research Information}"
    Power management and mainframe terminal: "{232}{}{FEV Summary Digest: 2073. As China became increasingly aggressive with their use of biological weapons, the United States government felt that a countermeasure was needed. The Pan-Immunity Virion Project (PVP) was officially formed September 15, 2073. }"
    "{233}{}{2075. It became clear that the best way to combat the newly created biological weapons was to alter uninfected DNA so that it was no longer susceptible to standard viral infection.}"
    "{234}{}{2076. Unforseen side effects began surfacing in early 2076 with the PVP. Animal test subjects began showing an abnormal growth rate accompanied by increased brain activity. }"
    "{235}{}{The U.S. government took notice of these discoveries, and in the interests of national security, moved a team on-site to secure and oversee the project, which was now dubbed the FEV (Forced Evolutionary Virus) project.}"
    "{236}{}{2077. FEV nears completion. Test on lab animals are at a near 100% success rate. Size and muscle density increase approximately 60%, and the protential intelligence increase by 200%. }"
    "{237}{}{Effects upon human subjects remain unknown; although they are theoretically promising. }"
    "{238}{}{The military, wishing to continue further testing, builds a large facility at the Mariposa military installation in central California. At this new facility, testing of the FEV virus continues on volunteer subjects from the military.}"
  3. FEV experiment disk: "Log Date January 7, 2077: Major Barnett has ordered transfer of all FEV research to the Mariposa Military Base. He plans to continue the project experiments on volunteer subjects. I am against this, and would like it noted here that research on human subjects is not recommended by myself or my staff."
  4. 4.0 4.1 Captain Roger Maxson's diary, October 10, 2077: "I, Roger Maxson, Captain, serial number 072389 have started this log because it doesn't look good for any of us, and I'd like for people to know what really happened here. All hell broke loose when we finally discovered what those scientist bastards were up to. The Colonel has locked himself in his office and seems to be having some sort of breakdown. The men are screaming for blood. They're looking to me for answers, and I'm not sure what to do. Someone has to do something, though, before this place sinks into an anarchistic bloodbath."
  5. Captain Roger Maxson's diary, October 12, 2077: "I stopped one of the men from executing a scientist today, and demanded that we interrogate them to find out what their orders were."
  6. Captain Roger Maxson's diary, October 13, 2077: "I killed a man today. I was interrogating Chief Scientist Anderson and he was giving me the full details of their inhuman experiments. He said his orders came from the Gov't., but I didn't buy it. He started screaming about how he was following orders, how he was a military man, and I just shot him..."
  7. Captain Roger Maxson's diary, October 15, 2077: "I broke down the door with several of the men just in time to watch him blow his head off. Right before he pulled the trigger he said he was sorry."
  8. Captain Roger Maxson's diary, October 18, 2077: "...The interrogations invariably end up being executions. Shellman held out the longest, but the end result was the same. Her arguments about her orders were a bit too specific to be completely made up. I'm getting a real bad feeling in my gut about how this is all going to end up..."
  9. Captain Roger Maxson's diary, October 23, 2077: "...Luckily, we had moved our families from outside into the facility the day before yesterday. We do not yet know if the fallout has reached this area..."
  10. 10.0 10.1 Cite error: Invalid <ref> tag; no text was provided for refs named MaxDiary
  11. Maxson log
  12. Captain Roger Maxson's diary, October 25, 2077: "Sgt. Platner volunteered to go outside today to take specific readings on the atmosphere. It seems the radiation has not spread this far. Since he was wearing his power armor, there was no threat to him from radiation, but if he had been exposed he would have had to be exiled. We don't have adequate decontamination facilities here."
  13. Captain Roger Maxson's diary, October 27, 2077: "We're leaving this godforsaken place today. I'm leading the exodus to the old government bunker at Lost Hills..."
  14. Fallout Bible 0: "2102 May 22 Increasing mutant attacks on Harold's caravans cause Harold to get so pissed he finances one of the first adventuring parties of Fallout to try and find out where these dagnab mutants are coming from. Consulting with a scientist and doctor at the Hub, a man by the name of Grey, the two of them decide to join forces."
  15. The Vault Dweller: "{128}{}{Where were the mutants coming from?}"
    Harold: "{133}{HROLD10}{Everywhere! Hell, seemed like you couldn't fart without hitting one. But mostly in the northwest.}"
    The Vault Dweller: "{134}{}{Did you explore that area?}"
    Harold: "{138}{HROLD11}{We mounted an expedition. God, Richard. Richard Grey. Led a small group of us up there.}"
    The Vault Dweller: "{139}{}{Richard Grey?}"
    Harold: "{146}{HROLD14}{Richard Grey was a doctor. Little older than me, and friend was he smart. He found the source.}"
    The Vault Dweller: "{147}{}{And what was that?}"
    Harold: "{160}{HROLD18}{Some sort of old military base. We lost a lotta folks getting in there.}"
    The Vault Dweller: "{161}{}{How did you know this base was the cause?}"
    Harold: "{165}{HROLD20}{Because it was like someone went bargain shopping at mutantland! Geez! Cheaper by the dozens! Can't figure any other reason except that being the factory.}"
    The Vault Dweller: "{166}{}{Did you examine the base closely?}"
    Harold: "{167}{HROLD21}{We got pretty far inside. Wasn't a lot of us left by then. Grey, me and couple of others.}"
    The Vault Dweller: "{168}{}{And then?}"
    Harold: "{169}{HROLD22}{One of them robots got Francine. Mark was wounded, sent him back to the surface. Then it was just me and Grey.}"
    The Vault Dweller: "{170}{}{What ever happened to Mark?}"
    Harold: "{172}{HROLD23}{To this day, I don't know. He never made it back here, and...well I...couldn't face the wasteland again, so I...I never looked.}"
    The Vault Dweller: "{173}{}{What about you and Grey?}"
    Harold: "{174}{HROLD24}{We made it to some sort of central core, like a plant of some sort. That's when it happened.}"
    The Vault Dweller: "{175}{}{What?}"
    Harold: "{176}{HROLD25}{A robot crane crashed into us. Last I saw of Grey, he was flying through the air in-into some sort of acid bath. I was in bad shape and...well I passed out.}"
    The Vault Dweller: "{177}{}{How did you survive?}"
    Harold: "{181}{HROLD26}{Well, I have no idea. Woke up in the wasteland, barely hanging on. Got lucky and some traders I knew found me days later. Good thing, since I was already changing. They brought me here, and here I've been since.}"
    The Vault Dweller: "{182}{}{How did you mutate?}"
    Harold: "{190}{HROLD28}{All's I know, is it was something inside that base.}"
    The Vault Dweller: "{191}{}{Do you think it could have been radiation?}"
    Harold: "{193}{HROLD29}{How the hell should I know? Grey would've known if anybody could. [sigh] Anyway, that's how the deal went down.}"
  16. Richard Grey's audio diary: 0154 - 0172: "I've begun to modify myself to be more pleasing to the Unity by injecting small doses of the virus into my body. The slime in the vats is a man-made virus called the Forced Evolutionary Virus. This information was acquired from my newly grown neurolink with the base computer."
  17. Fallout Bible 0: "2155-2156 After capturing a caravan of strange-garbed travelers (vault dwellers), Master learns the location of the Boneyard Vault, the future site of the Cathedral. He conquers the inhabitants and sets up operations there, and the human cultists begin to use the Vault as their powerbase. Within the Vault, the Master learns of other Vaults, and realizing their human occupants are ripe for transformation, begins to send out patrols to Vault locations in search of these other Vaults."
  18. The Vault Dweller: "{125}{}{What is it you do here?}"
    Lieutenant: "{129}{LIEUT7}{My dear human, this is the great procreator! Here we'll make others of the master race and insure the Unity. It's all quite glorious, I assure you.}"
    The Vault Dweller: "{131}{}{Sounds like it. Why do you need all this to, uh, procreate?}"
    Lieutenant: "{133}{LIEUT8}{Presently, there is a slight . . . problem in the reproductive process. It's being attended to. For now, we must use the Vats to turn humans into Super Mutants. You'll experience that glory yourself momentarily.}"
  19. Fallout Bible 0: 2. Is military base part of enclave or something? (sorry i'am not good at english :) but I think that you know what i wanted to say) - Deadlus
    "Unknown. The Mariposa Military Base was constructed for the purposes of FEV experimentation on human beings, and considering the nature of the "volunteers" (military prisoners who didn't have their brains scooped for use in brain bots) and the lack of any shred of ethics in the experimentation procedures, it is possible the Enclave had something to do with the experiments at Mariposa. In Mariposa records, however, the Military Base is never mentioned as under the direction of any organization called the "Enclave," and Colonel Spindel, head of the military squad stationed at the base, never indicated any Enclave allegiance... nor did Chief Scientist Anderson in the last few minutes before Maxson put a bullet through his skull."
    "Still, the existence of the Mariposa Military Base was listed in Enclave records, and this enabled the Enclave to find the base and begin their excavations, so it is possible that some elements of the Pre-War Enclave had their fingers in the horrors taking place at Mariposa. They held the site for many years, but abandoned it after obtaining the FEV samples... and noting the high incidence of mutation among the worker slaves and some of their soldiers, including Frank Horrigan."
  20. Fallout Bible 0: "2236 July 20 Enclave scouts discover the remains of the Mariposa Military Base and find it partially destroyed."
  21. Fallout Bible 0: "2236 July-August Enclave scientists and chemical corps scour the remains of Mariposa, while assault squads comb the desert for slaves they can use to mine the military base and get to the Vats. One of the squads includes soldier Frank Horrigan, 25 at the time, recently removed from duty on the President's secret service to take some RNR time in the wastes after some undocumented psychotic blunder or another."
  22. Fallout Bible 0: "2236: August Melchior is captured by an Enclave patrol and becomes part of the slave mining force at the Military Base."
  23. Fallout Bible 0: "2236: September Enclave construction crews and super mutant slaves begin excavations. They uncover the FEV virus, and mutations begin to occur in the human workers. Frank Horrigan comes into contact with the FEV and is sent to the Enclave labs for study."
  24. Fallout Bible 0: "2236 October Melchior begins to mutate... but keeps his intelligence and cunning in the wake of the transformation, making him pretty smart for a super mutant. Realizing that the Enclave will kill the super mutants after they get the FEV data, he begins to use his magician talents to secret away weapons for the mutants to defend themselves when the Enclave decides to dispense with them."
  25. Military base outside: "As expected the worker population has mutated rapidly once exposed to the pure source. Heavy, but acceptable losses are reported. I've ordered the evacuation of the base. As soon as the last person is out, we'll blow the entrance to the base and seal the bastards in."
  26. Dipping cutscene.
Mariposa Military Base