The lake Kleifarvatn is about 10 km². It is the largest of Reykjanes peninsula and the third largest of southern Iceland. It is about 97 m deep and one of the deepest lakes in Iceland. It is located on the fissure zone of the Mid-Atlantic Ridge. Their movements, as they pull apart, are what cause this region to have so many eruptions and earthquakes.
The lake has no visible water coming in or going out as most of its water comes and leaves underground. Its catchment area is small and it has a very limited discharge on the surface. The lake has diminished since year 2000 because of two major earthquakes, which probably opened up fissures at its bottom. In the sixties char fries from Lake Hlidarvatn were released into the lake and have thrived quite well.
In the southernmost part a hot water from some hot springs runs into the lake but elsewhere the lake is very cold. A small fishing lodge is located by the lake. Great place for photographers because of the volcanic surroundings of the lake are unique and beautiful. The story says that a monster in the shape of a worm and size of a medium sized whale lives in the lake.