An endorheic basin (from the Ancient Greek: ἔνδον, éndon, "within" and ῥεῖν, rheîn, "to flow") is a closed drainage basin that retains water and allows no outflow to other external bodies of water, such as rivers or oceans, but converges instead into lakes or swamps, permanent or seasonal, that equilibrate through evaporation. Such basins may also be referred to as closed or terminal basin or as internal drainage systems.
Normally, water that has accrued in a drainage basin eventually flows out through rivers or streams on the Earth's surface or by underground diffusion through permeable rock, ultimately ending up in the oceans. However, in an endorheic basin, rain (or other precipitation) that falls within it does not flow out but may only leave the drainage system by evaporation and seepage. The bottom of such a basin is typically occupied by a salt lake or salt pan.
Endorheic regions, in contrast to exorheic regions which flow to the ocean in geologically defined patterns, are closed hydrologic systems. Their surface waters drain to inland terminal locations where the water evaporates or seeps into the ground, having no access to discharge into the sea. Endorheic water bodies include some of the largest lakes in the world, such as the Aral Sea and the Caspian Sea, the world's largest saline body of water cut off from the ocean.
Pangong Tso, Tibetan for "long, narrow, enchanted lake", also referred to as Pangong Lake, is an endorheic lake in the Himalayas situated at a height of about 4,350 m (14,270 ft). It is 134 km (83 mi) long and extends from India to Tibet. 60% of the length of the lake lies in Tibet. The lake is 5 km (3.1 mi) wide at its broadest point. All together it covers 604 km2. During winter the lake freezes completely, despite being saline water.