The first major event is Gordon surviving the Citadel explosion at the end of the second game. Some calculations suggest this explosion was extremely powerful. However, it is implied that the G-Man intervened to take Gordon away from the blast, while Alyx was teleported away by the Vortigaunts, which she later credits for her survival. Although they were found in rubble, suggesting they survived the explosion, the context and significant intervention beforehand make it more likely that they were teleported away from the blast and ended up in the rubble due to the Vortigaunts. The other major instance is Gordon surviving the destruction of City 17 at the end of Episode 1. The blast can be reasonably assumed to have a city-level range, but it has been calculated to be 1.73 gigatons of TNT, or small island level. Gordon was clearly damaged by it, as seen at the start of Episode 2. But it was only the Surface Area of the explosion. While explosions of this magnitude are generally...