At this spot in Iceland, Þríhnúkagígur Volcano, molten rock once flowed through a cavity in the ground, but after an eruption ended, the molten rock retreated, leaving behind an open chamber in the ground. The walls are oxidized red, from the heat of the molten rock inside interacting with air as the magma drained away. People today can head into this site - literally heading into the plumbing of a volcano.