Lava tubes aren't the prettiest caves, but they're impressive when you remember that they were made by molten rock. I saw my first lava tube in Hawai'i, but it's gone now, covered with more lava.   (Left) San Cristóbal, Galapagos Islands. The lava that made this tube must have poured out into the sea with great clouds of steam.  (Right, top) Floreana, Galapagos Islands. A very narrow lava tube. See the layers of later lava flows beside the opening? Maybe they flattened the narrow tube.  (Right, lower) Santa Cruz, Galapagos Islands. Lava tubes can be much larger. Here's the end of a hike through a quarter-mile long lava tube. Note the rough lava walls. In places, water seeping down was beginning to make stalactites!