The Vidgelmir lava tube is 2 km long and within a lava field approximately 52 km long, covering a total area of around 242 km2.It is one of a few lava tubes which are present within the Hallmundarhraun lava which was emplaced around 900 AD.The cave itself is unique for more than just its size, during the harsh Icelandic winters it forms beatuiful ice statalgtites/stalagmites which are said to hold families of ice elves! There are also very well preserved lava stalagtites further on into the cave. The reason that the tube is currently accesible is due to the collapse of the roof, giving the wide entrance which is seen in the photos!
For more information and photos, visit the tour website. If you are in Iceland I would certainly recommend a visit!
Lava stalactites form when lava level in a tube drops and lava which has stuck to the ceiling begins to droop down. As it does so it cools, therefore forming stalactites!
Below is an example from the Cassoni tube at Mt. Etna.