Military base ruins

The ruins of the Mariposa Military Base (simply called military base in the game) are what remains of a top-secret United States military facility located somewhere in the area of the Mariposa unincorporated community in California. It acts as a late-game dungeon, with plenty of loot to take away for the crafty explorer.

The Military Base can be found ten squares east and twenty-seven squares south of Arroyo.

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. On July 20th, 2236, Enclave scouts (re)discovered the ruins of the Mariposa Military Base. The Chemical Corps and other scientists began to scour the base, while assault squads began gathering slaves to use for excavating the base. In the process, they capture Melchior from the town of Redding. 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. 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. 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. 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.

Surface
The layout of the base is pretty much similar to that in Fallout, although there are some tents outside the base, packs of wolves, and the entrance to the base is blocked by rubble. To get in the base you will need to find the metal pole and put it on the rail cart ("Use on"). Next, you need to put some dynamite (can be found in the little shack right from the entrance to the base) on the rail cart (you need to right click and hold on the cart, and then choose the "bag" option; after that, simply click on the dynamite from the menu) and push it down. Doing this will net you 5000 XP.

Stronghold Level 1
When the player arrives to the first level of the base, the first thing he/she would notice are the Enclave soldiers' and mutant bodies lying around. The level is mostly filled with mutated rats, which can poison you if they bite, but can be easily dispatched. However, in one of the halls, there is a super mutant, Grundel, which will attack you, if you try to talk to him. Note that in the southwest corner (in the hall leading to the elevator) there is a broken power generator, which can be repaired for 1500 XP (this can be easily done if you have Vic in your party). Doing this will also turn on the lights in the base.

Stronghold Level 2
The second level of the base is filled with about 15 super mutants usually wielding power fists and flamers, although few of them use laser rifles and plasma rifles. In most of the lockers you can find money, stimpaks, some ammunition and most notably, in the east corner of the map, a set of T-51b power armor.

Stronghold Level 3
Like on the second level of the base, third level is also inhabited by super mutants which use laser rifles, plasma rifles and flamers. Also, there are some Mutated Beasts (Pig rats) which are weak, but you can get poisoned if they bite you. Note that there are Super Duper Mutants on this level, which have greater HP than "normal" mutants.

As for loot, you should pay attention to the first room after you exit the elevator. In the room is the locker, in which you can find a green memory module, which can be used to expand ACE's medical library, thus allowing you to increase your Perception by 1. Finally, in the southeastern corner of the map, there is an elevator, which takes you to the final level of the base, the vats.

Vats
The deepest level of the base houses one of the tougher enemies in Fallout 2, Melchior. He is standing in the northwestern corner of the map, surrounded by goo which has remained there since the Vault Dweller destroyed the FEV vats.

Melchior himself is not that tough, but his abilities are quite unique. From the hat (or more precisely, the goo), he can and will summon deathclaws, fire geckos, floaters and mutated mole rats. They are summoned in that descending order, and if you are strong enough to survive several waves of them, you are rewarded with some funny lines. He can have four summoned allies at a time, which can be bothersome, so it is best to dispatch him first, and then his "pets". This is quite challenging, even for experienced player with a good party, so you should be very cautious, and prepared to leave the scene of battle with 1-2 companions less. Melchior also guards a locker containing a PPK12 Gauss pistol.

Appearances
The ruins of the Mariposa Military Base appear in Fallout 2. The base itself is mentioned in Fallout 3, a Fallout: New Vegas loading screen, and the Fallout: New Vegas add-on Dead Money.