The Presidential sub level is primarily a subway tunnel that runs under the White House between the east and west wings. It is patrolled by several sentry bots and further guarded by automated turrets. The sub level is also infested with feral ghouls in a gridlock with the automated defenses.
After the first tunnel and continuing into the second, you will find a Senate employee ID in a crashed Metro train, next to a skeleton, which will help you when talking to M.A.R.Go.T.
In the metro car directly behind the one where you find the employee ID you will find one mini nuke and a laser pistol.
If you continue past the door that leads to M.A.R.Go.T. you will find an intact garden gnome holding a camera and two teddy bears tied to the rails in front of the Gnome. It appears as though the bears are being held hostage with a miniature sign saying "Help Us!"
There is a skeleton hanging from the ceiling in one of the tunnels. Look closely - ramps on either side of the car and a motorcycle forward of the skeleton. Most likely, the poor soul tried to jump the car in the center of the sewer and clotheslined himself on the light in the process.
Placed throughout the sub level are little bronze busts of the former Presidents of the United States; however the only bust intact is that of President Jefferson, showing an obvious correlation to the Jefferson Memorial, home of Project Purity.
The log entry AK197833 in the terminal in the small maintenance room quotes the song "Detox" from the heavy metal band Strapping Young Lad, in particular the lines : "I wish that I could sleep, and just get this over with, this is only high school bullshit", and "I'm lost, I'm freaking, and everybody knows, everyone's watching". the complete lyrics for the song can be read here. The name given for the engineer on this terminal is D.T., a reference to Devin Townsend, vocalist of Strapping.