Barney Calhoun reaches the surface and encounters the HECU. He manages to fight through them and reach Dr. Rosenberg. He and a team of fellow scientists were trying to escape until the marines caught them.
Calhoun rescues some scientists who have been locked in railway cars in Black Mesa’s classification yards. One of the scientists, Dr. Rosenberg (who is also a character in Decay), advises Calhoun that the entire facility is surrounded by Hazardous Environment Combat Unit Marines intent upon killing all base personnel to cover-up Black Mesa’s research into extradimensional exploration.
The only way to escape, Rosenberg explains, is to use an old prototype of the Lambda Complex teleporter; if it works, they can reach an obscure entrance to Black Mesa which (hopefully) has been overlooked by the Marines.