Kleifarvatn is the largest lake on the Reykjanes Peninsula. It is situated on the fissure zone of the Mid-Atlantic Ridge a short distance from the dramatic geothermal area of Seltún.The lake is incredibly deep, 97 meters (318 feet) at its deepest point. But, its unique feature is that it has no visible surface drainage, which means there are no rivers running to or from it. Thus, the water level only changes with the ground water. Following an earthquake in the year 2000, a fissure formed at the bottom of the lake that soon began to drain. It diminished by 20 percent. Gradually though, the fissure refilled, and the lake has returned to its previous levels.
Today, the Kleifarvatn area is gradually becoming a popular destination for hikers, joggers and bird watchers. Surrounding the lake is a comfortable trail where you can enjoy the dramatic and ever-changing landscape. The lake itself attracts the local anglers who like to fish for trout in the tranquil, colorful area.
The lake is believed to be inhabited by a monster. The serpent-like creature is the size of a large whale and has been spotted surfacing now and then.
The lake is the setting for the crime novel The Draining Lake by one of Iceland's most prominent crime authors Arnaldur Indriðason