Unlike the more commonly known limestone caves, lava tubes are caves which form within lava flows on the flanks of some volcanoes. When the volcano is active, a virtual "river" of molten lava is pulled downhill by gravity. The top of the flow will occasionally cool and form a solidified crust, with the lava continuing to flow underneath. After the lava drains out, it leaves behind the empty passage. If there happens to be an opening to the outside world, the tube can be entered and explored -- some extend for thousands of feet. Lava tubes provide a valuable opportunity to study a lava flow from the inside.
This photo shows a cave explorer, or spelunker, entering a lava tube called Subway Cave. This cave in Lassen Volcanic National Park, California, is named for its uniformly arched ceiling and flat floor, reminiscent of the shape of a subway tunnel.
This photograph appeared on NASA/USRA's Earth Science Picture of the Day site on July 25, 2004.