Beat the average price to unlock them
2K said it would add more games to its ongoing Humble Bundle featuring the likes of BioShock, The Darkness II, and Spec Ops: The Line and indeed it has. Folks who pay more than the average (approximately $8) will gain access to the X-COM: Complete Pack.
The pack includes Apocalypse, Enforcer, Interceptor, Terror from the Deep and UFO Defense. Pretty good way to experience the iconic sci-fi series back before the hyphen was dropped.