A deep underground city spreading below
Tokyo-3. Built using an pre-existing
spherical cavity (most of which was filled with earth and sand), the ceiling portion was almost totally destroyed by the
JSSDF's attack in episode 25' "
Air", thus exposing the
Geofront.
NERV HQ is located in the centre of the
Geofront, and is surrounded by
forests and a subterranean
lake. The many high-rise buildings that extend downward from the ceiling
(Ceiling City = TENJOU TOUSHI) are the same buildings that stand in
Tokyo-3, and are retracted
underground during
Angel attacks. The start of the
Instrumentality Project revealed that this cavity is actually the egg of
Lilith -- the "
Black Moon".
Source: The End of Evangelion : Glossary contained within The End of Evangelion - Theatrical Program.