There's a cool post up on boingboing.net about the deepest cave in the United States, located on the Great Plains in southeastern New Mexico. It's called Lechuguilla Cave and it wasn't even discovered until 1986! It is in the Carlsbad Caverns National Park, but you can't visit this particular cave. Only scientists and explorers can go in. However, you can take a tour without even getting out of your chair, because boingboing has posted this video of the cave from Planet Earth.