Víðgelmir
Encyclopedia
Víðgelmir is a lava tube
Lava tube
Lava tubes are natural conduits through which lava travels beneath the surface of a lava flow, expelled by a volcano during an eruption. They can be actively draining lava from a source, or can be extinct, meaning the lava flow has ceased and the rock has cooled and left a long, cave-like...

 situated in Western Iceland in the Hallmundarhraun lava field, ca. 2 km. Southeast from Fljótstunga farm in Hvítársíða, Borgarfjörður
Borgarfjörður
Borgarfjörður is a fjord in the west of Iceland near the town of Borgarnes. The waters of Borgarfjörður appear to be calm, however the fjord is on the contrary a rather dangerous part of the sea because of its undercurrents and shallows....

. The roof of the lava tube has collapsed, creating two large openings near its north end which are the only known entrances. Viðgelmir is 1585m long, the largest part of the cave passage is 15.8m high and 16.5m wide making it by far the largest of its kind in Iceland. The cave has a wide entrance but narrows down in some places. An iron gate was installed at the first constriction in 1994 to preserve the delicate lava formations or speleothems which haven't already been destroyed. Evidence of human habitation, probably dating to the Viking age
Viking Age
Viking Age is the term for the period in European history, especially Northern European and Scandinavian history, spanning the late 8th to 11th centuries. Scandinavian Vikings explored Europe by its oceans and rivers through trade and warfare. The Vikings also reached Iceland, Greenland,...

, has been discovered in the cave and is preserved in the National Museum of Iceland
National Museum of Iceland
The National Museum of Iceland was established on 24 February 1863, with Jón Árnason the first curator of the Icelandic collection, previously kept in Danish museums...

. Long stretches of the cave floor are very rough and shouldn't be navigated without a guide. Access and guided tours are provided at nearby Fljótstunga.

Lava tube caves are formed when a low-viscosity lava flow develops a continuous and hard crust which thickens and forms a roof above the molten lava stream. When the eruption subsides, the still molten lava moving beneath the crust will continue to drain downhill, leaving an open lava tube cave. Many other lava tube caves have been discovered in Hallmundarhraun (formed around 900 AD), most notable Surtshellir
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. While mentioned in the medieval historical-geographical work Landnámabók, Eggert Ólafsson was the first to give a...

 and Stefánshellir.

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