Mljet lies south of the Pelješac peninsula, from which it is divided by the Mljet Channel. Its length is 37 kilometres (23 mi); its average breadth 3.2 kilometres (2.0 mi). It is of volcanic origin, with numerous chasms and gorges, of which the longest, the Babino Polje, connects the north and south of the island. Port Polače, the principal harbour in the north, is a port of call for tourist ferries. Mljet contains one hotel—The Odisej (from the Greek Odysseus) in the north-west corner of the island.
The northwestern part of the island includes an inland sea as well as a small island within it. It has been a national park since November 12, 1960.
Over 84% of the island of 98.01 square kilometres (37.84 sq mi) is forest. The island’s geological structure consists of limestone anddolomite forming ridges, crests and slopes. A few depressions on the island of Mljet are below sea level and are known as blatine (“mud-lakes”) or slatine (“salt-lakes”). During the rain seasons all blatine are filled with water and turn to brackish during dry seasons.
The climate is Mediterranean; an average air temperature in January is 9 °C (48 °F) and in July about 24 °C (75 °F).