The Entrance to Hell is a great Flickr photo pool full of mysterious portals.
Take your own plunge into the underground in CHASM, a very simple but satisfying game in which you must pilot a ship through a deep, narrow tunnel. The only thing I could have wished for was an option to reverse the Y axis, as my fighter pilot instincts inevitably kicked in whenever things got hairy and I'd plunge straight up or down into a wall, completely intending to go the opposite direction. May you fare better than I.
Under New York is an urban exploration site dedicated to exploring the dark, abandoned parts of the city. Most of the sites involve the metaphorical underside of the city, but there are quite a few interesting subterranean expeditions under Tunnels & Bridges. While the pictures are small and too few, there are a few gems — most taken looking back at the world above.