The inner workings of Valve are shrouded in mystery, particularly when it comes to game development. Consider the unfinished game that Valve was working on at the same time as the original Half-Life: Prospero.
ValveTime.net took the time to unpack everything we know about the long lost game. They get into the nitty gritty about what it was about, its influences, and how it would foreshadow Valve games yet to come.
Valvetime also has several previously unreleased screenshots from the game over at their site. Check 'em out: