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 sentry bots and further guarded by automated machine gun turrets.
Senate employee ID - after the first tunnel and continuing into the second. It is found in a crashed metro train, next to a skeleton.
Mini nuke - in the metro car directly behind the one containing the senate employee ID.
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 with a motorcycle ahead of the body. Most likely, the man tried to jump the motorcycle 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."
2 intact presidential statues can be found in the sub level, 1 full size another in miniature.