Lake Ladoga (Laatokka in Finnish) is the largest freshwater lake by surface area that is entirely in Europe. It has been entirely within Russia since the Paris Peace Treaty of 1947. Its northwestern shoreline used to be part of Finland's territory until then. Ladoga is not very deep. Its average surface area is about 17,890 m2 and its average depth is only 51 m. The most notable place worth seeing is the Valaam Monastery. The Orthodox Christian monastery is on an island going by the same name.
Valaam Monastery
The Finnish monks left the island and the New Valaam Monastery was re-established in Heinävesi, Finland.