A massive subterranean spherical cavity approximately 6km wide and almost 1km tall, located directly under New Tokyo-3. At it's center stands NERV Headquarters, a giant pyramid surrounded by sprawling forests and a subterranean lake.
On the ceiling of the Geo Front, directly above NERV Headquarters, hangs what is called the Ceiling City. These upside-down skyscrapers are actually New Tokyo-3, whose structures are retracted underground during Angel attacks. The Geo Front is protected by 22 layers of reinforced armor.
Although the cavity itself is a natural phenomenon, the space inside it, including the forests and lakes, were artificially terraformed during the construction of New Tokyo-3. Car trains and monorails are used for transportation between the surface and the Geo Front.
In The End of Evangelion, New Tokyo-3 and the Ceiling City are destroyed by the Strategic Self Defense Force using Nē Weapons, exposing the Geo Front to direct bombardment. Shortly following the execution of the Human Instrumentality Project, the Geo Front is revealed to be the Black Moon, or Lilith's Egg, and the source of all human life.
Based on Neon Genesis Evangelion Platinum: 05 DVD Booklet