The Black Mesa base as depicted by VALVe is an actual representation of a particular underground base at Dulce in New Mexico. What's interesting is the fact that there is an underground rail system connecting the many subterranean bases. The region of Dulce is very similar to the surroundings at Black Mesa, and there are certain many entrances located at various Mesas, canyons, and mountainsides. Also interesting is the fact that Dulce base was said to have been the setting of a war involving extraterrestrials and military personnel. This became known as 'The Dulce Wars'. The human personnel were outraged by the atrocities being committed by the ET involvement against humans, such as the many genetic experiments that took place. One particular guy, Phil Schneider worked on the construction of the base and was said to have been hit by a laser weapon, which had scared an area on his chest. Phil Schneider was murdered in what was supposed to look like a suicide when he told of the base. A major base is located beneath Denver Airport, Colorado, and it is also said that many major structures in different regions have entrances to the underground networks, such as hospitals, military bases, freemasonic lodges, Mormon temples, churches, museums, government offices, etc.
For more information on underground bases visit these sites:
http://www.geocities.com/hyperdigm/tunnels.htmlhttp://www.angelfire.com/ut/branton/http://www.anomalies-unlimited.com/Denver_Airport.htmlhttp://www.pastpresentfuture.info/alien-ufo-secret-dulce-base.html