A couple of years ago, I wrote an article about how future space explorers may live in lava tubes on the moon and Mars. So it was of particular interest to me to walk through this lava tube on the island of Santa Cruz in the Galapagos. The tube was over half a kilometer long, I'd say, and had a diameter about that of a subway tunnel. More pics from our Galapagos trip here.
UPDATE: Scientific American has chosen to delete its "community" section, so one of the links above will not work.