During long eruptions, lava flows can be channelized into a few main streams. Flows that persist for many hours can solidify on the top and sides of the stream, leaving a tube for the liquid lava to flow through. The solidified lava is a good thermal insulator, so the lava flowing inside remains liquid much longer than an exposed flow. Lava tubes can transport lava several miles from an eruption site.
After the lava has drained from the tube, a lava tube cave is left. These caves are often big enough for people to walk through, and can cover long distances.