Þingvallavatn, at 83,7 km2, the largest lake in Iceland. Its surface is 103 m above sea level, its average depth 34,1 m, but it reaches a depth of 114 m, i.e. 11 m below sea level. Fed largely by underwater springs the water is always extremely cold, the only river feeding it being Öxará, and the river Sog being its outlet.
Two islands, Sandey and Nesjaey, both old craters.
Many summer houses around the lake.