Huh TIL that Source's physics engine was implemented all the way back in 1999, they licensed Havok before it was even bought by Havok, it was called Ipion Virtual Physics at the time
Also apparently Source 2 ditched Havok for Valve's custom engine though, and I wonder how much of that is Valve building on top of Havok like Bethesda's Creation Engine building on top of Gamebryo, and how much is actually completely original