Nearly twenty years later, Half-Life 2 opens as the G-Man brings Freeman out of stasis and inserts him into a dystopian Earth ruled by the Combine, a faction consisting of human and alien members, that used the dimensional rift caused at Black Mesa to conquer Earth in the interim. Barney Calhoun, a security officer, also escaped from the facility with Dr. The G-Man then destroys Black Mesa with a nuclear warhead, and detains Shephard in stasis. Back in Black Mesa, a second alien race begins an invasion, but is stopped when a Marine corporal, Adrian Shephard, collapses its portal in the facility. On Xen, Freeman eliminates the alien 'leader' and is confronted by the G-Man, who offers Freeman employment before putting him into stasis. Freeman fights through the facility to meet with several other scientists, who decide to travel to the alien dimension to stop the aliens. Attempts by the Black Mesa personnel to close the rift are unsuccessful, leading to a Marine Recon unit being sent in to silence the facility, including any survivors from the science team. Gordon Freeman, a recently employed theoretical physicist, is involved in an experiment analyzing an unknown crystalline artifact however, when the anti-mass spectrometer beam contacts the crystal, it creates a resonance cascade that opens a dimensional rift between Black Mesa and another world called Xen, causing monsters to swarm Black Mesa and kill many of the facility's personnel. The Half-Life series begins in the 2000s, at the fictional Black Mesa Research Facility in New Mexico. An American scientific research facility opens a portal to a hostile alien world, who begin to invade the earth.

What is a half life