On the southern part of the peninsula, within the Reykjanes Nature Preserve, lies Lake Kleifarvatn, a starkly beautiful body of water with black sand beaches. Both large and deep, it exudes an eerie atmosphere with barren, volcanic surroundings. The lake is the largest one on the Reykjanes peninsula. 

There are no springs or rivers that feed lake Kleifarvatn nor is there a river or a creek going out it. Most of it's water enters and leaves via undarground tunnels. In recent years the waterlevel has dropped, after an earthquake in the year 2000, and though is gained almost it's original size in 2008, it seems to be receding again. It's almost 100 meters deep and there are many picturesque places around it.  

Best viewed from Lambstangi spit on the southern end, char fishing is available during the summer months. Be on the lookout for the resident Kleifarvatn monster, a legendary worm-shaped aquatic beast the size of a whale swimming around the lake. The lake also plays a pivotal role in one of Arnaldur Indridason's, Iceland's most populare crime fiction novelist. The novel bears the same name as the lake, though in english it's called The Draining Lake. In the novel the draining of the lake reveals a body lon hidden in the lake and detective Erlendur is called upon to solve the mystery.