As Gordon rides the tram into the Materials Transport section of Black Mesa, he learns from a security guard that he must ride to the High Altitude Launch Center to launch a satellite, which the Lambda Team can use to reverse the effects of the Resonance Cascade.
Freeman also discovers the HECU's true hatred for him, as well as their larger reinforcements: sandbag bunkers with rapid-fire sniper rifles, mounted machine gun turrets (manual and automated), and even mounted rocket launchers.
After going round in circles for a while, Gordon goes up several elevators and at last reaches the surface again near the launch site; he sends the rocket up into orbit and goes back underground.
