Surtshellir

right|thumb|One of four entrances to the Surtshellir lava tube in the Hallmundarhraun lava field. right|thumb|Looking up towards the fourth (lowest and farthest west) entrance to Surtshellir, seen from within the section of the cave called Íshellir (ice-cave). The white slab is ice, which was preserved into summertime. thumb|Typical passage shapes in the Surtshellir-Stefanshellir lava tube system, showing intact walls and pahoehoe floors Surtshellir () is a lava cave located in western Iceland, around 60 km from the settlement of Borgarnes. Approximately a mile in length, it is one of the longest such caves 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.

Surtshellir was formed as a lava tube in a massive, effusive, shield-volcano eruption at the northern end of the Prestahnúkur volcanic system. At least three and possibly four vents opened on the edge of Langjökull glacier during this long-lived eruption, which has been dated to the first decades of Iceland's settlement (c. 880-920). Ultimately, the eruption covered 240 km2 (90 mi2) of once fertile grazing and settled land beneath sheets of lava 20 meters thick, or more. Surtshellir is one of 20 known caves in the Hallmundarhraun lava field that once carried magma from the vents to the farthest ends of the lava field, more than 50 km to the west. Eight of these (including Viðgelmir and Hallmundarhellir) are known to contain archaeological remains of varying age; Surtshellir's are the most extensive.