The Black Mesa incident resulted in hundreds, if not thousands, of deaths and is still effecting humanity to this day. So what happened and how was Gordon Freeman one of the only people to escape the science facility?
Everything started when Xen crystal sample "GG-3883" was pushed into the Anti-Mass Spectrometer, causing a Resonance Cascade. This caused a massive explosion in and around the test chamber, killing most people near by. All sorts of wildlife from Xen was teleported all over the facility