Looking down the axis of a classic lava tube passage, Lava Beds National Monument, California.Park service cross-section map showing lava mold caves, Mt. St. Helens National Monument in Washington.
Thurston Lava Tube, Hawaii Volcanoes National Park. The step mark on the right wall indicates the depth at which the lava flowed for a period of time. And as anyone who's seen the movie "Surf's Up" knows, you can surf through lava tubes as well! BTW, there is an article on the tunnels where Jughead was kept, in the latest Lost magazine, including set design photos. I would have included some scans here, but the ones that were posted by ODI were removed as per ABC's request.