Deep Sea is a game created by Robin Arnott, also known for his brilliant sound work on games like Capsule and Antichamber. It's a game with no screen; in fact, the player is blinded by wearing a gas mask while playing. It uses only binaural sound and a joystick to allow the player to move around the ocean floor. The only sounds are the player's breathing patterns and "the rumbles made by unseen terrors."
If you're into getting the pants scared off you, Mr. Arnott himself has put together a tutorial on how to create your own mask over at TechHive. You're going to need to buy a few things you likely don't have lying around the house, but I'd be lying if I said I wasn't at least a little bit interested in trying it out for myself. Unfortunately, the first step of owning a Mac threw all my hopes out the window.