Lake Kleifarvatn is the biggest lake on the Reykjanes peninsula, just 30 minutes drive outside of Reykjavik. Kleifarvatn is close to 9 km2 and one of the deepest lakes in the country reaching more than 90 metres down.
The lake exists because of ground water level since there is no river flowing into it or out of it. After some strong earthquakes in 2008 the water level collapsed and nobody knew why until the scientists found out that some cracks had opened up on the bottom of the lake. It took the water several years to flow into the cracks and fill them up with water. The water level is nowadays back to normal.