A lava tube forms when treacle-like basaltic lava flows downhill from a volcano along a channel like a river. Over time, a solid rock crust forms on the channel’s surface as the 1,000-degree-Celsius (1,832-degree-Fahrenheit) lava cools when it’s exposed to air. The lava within can remain hot and runny for tens of kilometres even when the tube is completely crusted over.
The ropey-looking lava emerging from the tube is called pahoehoe – a Hawaiian word for flows that form bizarre shapes. The tube is only partly filled by lava: the lava’s heat downcuts through the channel bed. Superheated air and gas fill the space above the lava and re-melt the ceiling to create soda straw stalagmites – formations which are only found in lava tubes.