In an interview with The Steamcast, Gabe Newell revealed that the company had been working on a game, code named Prospero (after the Shakespearean character in The Tempest) which was slated for development and release prior to Half-Life but never went anywhere.
Newell said that a lot of the concepts that went into its development influenced their other subsequent games, such as they are. While Valve has no plans to lift the concept from the bottom of the sea, Newell said that the game holds a special place in their hearts and that nothing at Valve ever really dies as concepts from abandoned projects are often used in ones that do make it out the door.
"Nothing every really dies here, it's just a question how long until we get around to doing it. We just have so many opportunities to do so many different things. I mean I even get emailed from people saying 'What about Ricochet 2?', which is kind of astonishing.
"We actually have twenty six people whose first Valve game they every bought was Ricochet on Steam, so like I said nothing ever appears to ever really die, you just haven't gotten around to doing it yet. I'd put Propsero into the camp. And even further back than Propsero, the other game that Mike [Harrington] and I were talking about before we'd even had anybody other than us working on it was a submarine game.
"Mike was absolutely certain there was an opportunity to create fantastic underwater visuals and gameplay, so if people want to know the even furtherest back thing we have haven't gotten around to shipping yet it would have to be the unnamed submarine game."