Goplo, a shallow postglacial flowing lake, situated in so called Goplo postglacial channel, is one of the well-known natural water bodies in Poland. It is the biggest lake of the Greater Poland-Kuyavian lake district and the 9th biggest lake in Poland. It is said that the water table at 77m.a.s.l. is 2,154.4ha in area; the islands are 25.5ha out of the area. The lake basin is 25km long and 200m up to 2000m wide. The average depth of the lake is 3.6m and the maximum depth is 16.6m. Goplo has a varied 87.5km long shoreline, which consists of numerous islets, bays and peninsulas.
In the past, the lake was much bigger in area, and its water table was called (according to the chronicle by Jan Dlugosz) Mare Polonorum, which means The Poles’ Sea.