Behind us is the entrance of a vacant Lava Tube. Lava tubes are natural tunnels where lava flows underneath the ground, in may cases for miles. When the eruption is finished the tubes run dry and leave behind tunnels that look man-made. This is caused by low viscosity lava. It forms a hard crust that gets thicker and thicker, while lava is flowing inside it.
It was mind boggling to walk through something that once was filled with liquid rock.