Kleifarvatn is the largest lake of the Reykjanes peninsula. It is the third largest lake of southern Iceland, 9,1 km², and one of the deepest lakes of the country, 97 m. Its catchment area is small and it has no discharge. Consequently it varies in size during the year. Char fries from lake Hlidarvatn were released into it in the sixties and have thrived quite well. The best catch can be expected in the southernmost part of the lake where hot water from some hot springs runs into it.
Elsewhere the lake is very cold. There is a small fishing lodge by the lake. The barren, volcanic surroundings are uniquely beautiful. During 2000, 2001 and 2002 the lake was considerably reduced in size in the wake of an earthquake period, which made its bed more permeable.
In 2007, sedimentation sealed the lake floor for the most part, and almost constant precipitation between August and December filled the lake basin again. In January 2008 the lake level had more than recovered.