Surtshellir is the longest and most renowned cave in Iceland. It is located a short distance north of Mt. Strutur and about 7 km from the farm Kalmanstunga. The total length of the cave is approximately 1970 m. The cave, Stefanshellir, is a direct continuation of Surtshellir to the northeast, and together they are by far the longest caves of the country, 3500 m.

The roof of Surtshellir has collapsed in five places, where there are five openings, one of them small. As a rule, the northernmost opening is used as an entrance. A bit further to the south, there are two big openings and the southernmost one is only about 400 m short of the end of the cave.

The average height of the cave is about 9 m and at the end of the cave, it drops to 2-4 m. A part of the cave shows signs of human habitation in the past, some bones, ashes, stonewalls and bedsteads made of stone. The innermost part, where the ceiling is lower, is called "The Ice Cave", where one usually sees remarkable ice formations, ice candles and columns, sometimes all the way up to the ceiling. This part of the cave is the most easily passable part, but also a very slippery one, and the most beautiful as well.