Peripeteia is an incredible game for getting lost in. You can spend hours wandering around and climbing its monolithic BSP level editor architecture, and all the while finding new vistas, gear, and weird little areas in places that'd be out of bounds in any other game. It doesn't hold your hand, doesn't "subtly" guide you with standard level designer tricks, and in return you get a phenomenal sense of discovery out of it, as it truly feels like it's *you* finding all this neat stuff and not the level designer pulling you along on a string.
People sink tens (if not hundreds) of hours into the demos for this thing, and the full game isn't even out yet. It's nuts.