The Core is a total conversion single-player mod for Half-Life. Survive the disaster as Maurice Andrews, a low level engineer stationed in Black Mesa during the events of the resonance cascade. Seemingly trapped, Andrews must fight his way to the surface to survive.
The Core on ModDB

Press
Kotaku: It's Amazing What Modders Can Do with the 15-Year-Old Half-Life
PC Gamer: Half-Life mod The Core releases first trailer, shows we're still not done with Black Mesa