The access tunnels are a small set of tunnels under the unfinished reactor in the Gecko junkyard. It is the home of the Renewal Cult, a small band of ghouls who believe in the teachings of the Brain. It is accessed through a manhole in the rear of the shack built onto the reactor building.
It is a small group of natural rock caves and man-made reactor tunnels. The Brain is located in its deepest section.
The access tunnels appear only in Fallout 2.