The latest Game Informer has Call of Duty: Advanced Warfare as its cover story, but it also revealed a bit about developer Sledgehammer's earlier, cancelled project. Their first Activision project was a third-person shooter that was set during the Vietnam War, with a focus on the Secret War in Cambodia.
Game Informer's talk with co-founder Glen Schofield revealed that they spent up to eight months on that game, and had it to the point to where they had some cool mechanics. There was about 15 minutes of playable content before the game was cancelled and the team was reassigned to Modern Warfare 3.
Parts of this cancelled game took place in some underground tunnels. The interview revealed that the team was going for some Dead Space style moments. This sounds like it would have been a pretty interesting game!