Vidgelmir ( Isl. Víðgelmir ) is a lava tube located in the west of Iceland on the Hadlmundarhrein lava field, about 2 km southeast of the Floutstung farm ( Isl. Fljótstunga ). The cave arch collapsed, creating two holes from the northern edge of the tube, which are the only known entrances to the cave. Vidgelmir is 1585 m long, the largest part of the passage reaches a height of 15.8 m and a width of 16.5 m. The cave is wide at the entrance, but in some places it narrows.
There is evidence that the cave was inhabited during the Viking Age .
Lava tubes form when a low-viscosity lava forms a continuous hard crust, which, thickening, creates a vault over a stream of liquid lava. When the eruption ceases, liquid lava flows under the crust and, flowing down, leaves an empty lava tube. Other famous lava pipes in Iceland include the Surshedlir and Stevaunshedlir.