Found throughout the Big Island of Hawaii, lava tubes and caves are perfect examples of how nature forms itself. Lava caves like the one in the picture above are formed when a river of lava gradually builds solid walls and a ceiling as it is flowing. When the lava flow stops and the last of it passes downhill and into the ocean, a tube/cave is left. These caves can be a few feet high and only yards long, or they can stretch for miles with high ceilings.
Having explored some of these myself, I find it fascinating how the lava cools around itself and forms these kind of gravity defying spaces that create networks of tunnels under the surface of the islands.