Surtshellir

Surtshellir is a lava cave located in western Iceland, around 60 km from the settlement of Borgarnes. Approximately a mile in length, it is the longest such cave in the country. It was the first known lava tube in the world, at least by modern speleologists, and remained the longest known lava tube until the end of the 19th century. While mentioned in the medieval historical-geographical work Landnámabók, Eggert Ólafsson was the first to give a thorough documentation of the cave in his 1750 travels of the region. It is named after the fire giant Surtr, a prominent figure in Norse mythology, who is prophesied to one day engulf the world in the fire of his flaming sword.
Being of volcanic origin, the walls of the interior are composed of vitrified layers of magma and basalt. The roof of the cave is about 10 metres high at the highest point, and the tunnels are around 15m broad at their greatest width. The floor is covered in a perpetual sheeting of ice and fallen fragments of solidified lava, and large ice speleothems are common within the cave. The height of the roof is highly variable throughout the cave, and at the latter extremities is only 2–4 m.

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