For the mission involving this location, see Alien Base Assault.
Alien Base.jpg

XCOM discovers the high likelihood of an Alien Base hidden somewhere on Earth after capturing an Outsider Shard. After studying the shard, XCOM is able to identify a communication frequency that allows them locate the underground base; the Alien Base is located on a random continent for each game.

In addition to serving as a command and control center and supply depot for the invaders, the Aliens have been transferring abducted humans here to experiment on them.