At the end of the Highway to Hana, a famous scenic drive on Maui, is the Hana Lava Tube, KA`ELEKU CAVERNS. Unlike caves which can take millenia to form, lava tubes can form very quickly, over a few weeks. As massive rivers of lava flows from a volcano the lava on the outside of the thick river of lava will cool occasionally hardening to form a tunnel. When the lava stops flowing you are left with a circular lava tube running for up to 30 miles!
Like many tubes left behind after lava flows, the Lava tube on the Hana road of Maui shows all the geologic elements of lava flows that crust over. Among the many interesting things found in a lava tube are tube slime, extreme bacteria that live on the nitrogen, lava balls, which form much like a snowball rolling down a snowy hill, and lava falls, where a flow of lava has been frozen mid fall.
One enters the tube through a hole in the tubes ceiling created by gas punching through the ceiling of the lava tube. Hana Lava Tube has self guided tour on an unlit path. Don't wander off though, the tunnels often have maze like "braided" smaller tunnels in which it is easy to get lost.