The Parliamentary Plains are the most important historic site of the country. They are located just below the western fault line of the Icelandic Graben within the boundaries of the first National Park, which was established in 1930. The oldest parliament of the world was founded here in 930 on Lake Thingvallavatn, the largest natural lake of the country with an area of 83,7 km². Its greatest depth is 114 m, and this lowest point lies 13 m below sea level. Its discharge is River Sog, the longest spring fed river of the country with a very constant volume of 112 m³/sec. Three hydro electric power stations are situated on the river, which counts among the good salmon rivers. Very little water enters the lake on the surface, which means that it is mainly spring fed. The natural surroundings are majestic and the contrasts are great. There is a system of interesting hiking trails all over the park. The present natural landscape has been develloping for about 9000 years and is still being shaped by the plate tectonics. Source : http://www.nat.is/travelguideeng/thingvellir.htm