is hydrothermal activity throughout the region. For much of its history, until the Pleistocene, Lake Van has had an outlet towards the southwest. However, the level of this threshold has varied over time, as the lake has been blocked by successive lava flows from Nemrut volcano westward towards the Muş Plain. This threshold has then been lowered at times by erosion. Bathymetry The first acoustic survey of Lake Van was performed in 1974. Kempe and Degens later identified three physiographic provinces comprising the lake: a lacustrine shelf (27% of the lake) from the shore to a clear gradient change a steeper lacustrine slope (63%) a deep, relatively flat basin province (10%) in the western center of the lake. The deepest part of the lake is the Tatvan basin, which is almost completely bounded by faults. Prehistoric lake levels Land terraces (remnant dry, upper banks from previous shorelines) above the present shore have long been recognized. On a visit in 1898, geologist Felix Oswald noted three raised beaches at 15, 50 and 100 feet (5, 15 and 30 meters) above the lake then, as well as recently drowned trees. Research in the past century has identified many similar terraces, and the lake's level has fluctuated significantly during that time. As the lake has no outlet, the level over recent millennia rests on inflow and evaporation. The water level has vacillated greatly. Investigation by a team including Degens in the early 1980s determined that the highest lake levels ( above the current height) had been during the last ice age, about 18,000 years ago. Approximately 9,500 years ago there was a dramatic drop to more than below the present level. This was followed by an equally-dramatic rise around 6,500 years ago. As a deep lake with no outlet, Lake Van has accumulated great amounts of sediment washed in from surrounding plains and valleys, and occasionally deposited as ash from eruptions of nearby volcanoes. This layer of sediment is estimated to be up to thick in places, and has attracted climatologists and vulcanologists interested in drilling cores to examine the layered sediments. In 1989 and 1990, an international team of geologists led by Stephan Kempe from the University of Hamburg retrieved ten sediment cores from depths up to . Although these cores only penetrated the first few meters of sediment, they provided sufficient varves to give proxy climate data for up to 14,570