With an area of about 30 km2 and a depth of up to 58 m, the Großer Plöner See is the largest and deepest lake in the state of Schleswig-Holstein.

It is a typical Weichselian glacial lake known as a Zungenbecken, whose shape may be especially well seen on the southern shore between Bosau and Nehmten.

The eastern and the western parts of the lake are connected by an area of shallow water which was designated in 1992 as a nature reserve.

The Princes' Island (Prinzeninsel), which became a peninsula in the 19th century when the water level was artificially lowered, extends from here for 2 km to the north and is a popular summer destination for tourists.

Another nature reserve that has existed since 1955, is located in the western part of the lake around the Ascheberger Warder.

In 1139 the Holstein count Adolf II of Schauenburg destroyed the fort and ended Slav rule over the Plön region.

Species of fish include eel, perch, bream, pike, tench, carp, whiting, and whitefish.