The deepest portions of the hypolimnion have low oxygen concentrations. The outlet provides a laminar flow and a horizontal outflow into the hypolimnion. Heavy water exists in elevated concentration in the hypolimnion of Lake Tanganyika in East Africa. In eutrophic lakes, the hypolimnion is often anoxic. The lowest zone in the lake is the coldest and is called the hypolimnion. The warm layer is called the epilimnion and the cold layer is called the hypolimnion. The hypolimnion can be anoxic for up to half the year. The word hypolimnion is derived from the Greek "limnos" meaning "lake". Anoxia is more common in the hypolimnion during the summer when mixing does not occur. The hypolimnion may be much warmer in lakes at warmer latitudes. In the absence of oxygen from the epilimnion, decomposition can cause hypoxia in the hypolimnion. The hypolimnion, however, circulates very slowly and has no atmospheric contact.