Lake Nipigon is the largest lake entirely within the boundaries of the Canadian province of Ontario (since four of the five Great Lakes are split between the U.S. and Canada). It is sometimes described as the sixth Great Lake. Lying 260 metres (853 ft) above sea level, the lake drains into the Nipigon River and thence into Nipigon Bay of Lake Superior. The lake and river are the largest tributaries of Lake Superior. It lies about 120 kilometres (75 mi) northeast of the city of Thunder Bay, Ontario.
Lake Nipigon has a total area (including islands within the lake) of 4,848 square kilometres (1,872 sq mi) — compared to 3,150 square kilometres (1,220 sq mi) for Lake of the Woods. The largest islands are Caribou Island, Geikie Island, Katatota Island, Kelvin Island, Logan Island, Murchison Island, Murray Island, and Shakespeare Island. Maximum depth is 165 metres (541 ft). The lake is noted for its towering cliffs and unusual green-black sand beaches composed of the fine particles of a dark green mineral known as pyroxene. The lake basin provides an important habitat for woodland caribou.