|The following is based on Van Buren and has not been confirmed by canon sources.|
The Secret tunnel entrance is a location that was set to appear in the canceled Fallout 3 by Black Isle Studios, codenamed Van Buren. It is a secret entrance to Boulder Dome.
A secret, non-quarantined tunnel access point into the tunnels under boulder. Basically, a heavy manhole in the ground in the middle of a blasted open field.
Xian - found half dead in the tunnel.