Þingvallavatn, anglicised as Thingvallavatn, is a rift valley lake in southwestern Iceland. With a surface of 84 km2 it is the largest natural lake in Iceland. Its greatest depth is 114 m. At the northern shore of the lake, at Þingvellir, the Alþingi, the national parliament, was founded in the year 930, and held its sessions there until 1799 and still as of today the name Alþingi Íslendinga is carried by the parliament of Iceland.