Vatnshelli cave - West Iceland

"The cave itself is an 8000 year old lava tube created by volcanic eruption from a nearby crater in the Purkhólar crater family. As the lava rushed down the hill in a lava river it began to cool on the surface, creating a crust on top of the lava river. As the eruption stopped, all the lava from underneath this crust continued to drain out. That eventually left behind empty tube with roof on top that gradually cooled down."