Location:Republic of Karelia
Diameter:14 km
Age:700 ± 5 million
Coordinates:61° 58'N 30° 55'E
Link to Google Maps: here
Source: http://en.wikipedia.org/wiki/J%C3%A4nisj%C3%A4rvi

The basin of this somewhat circular lake was formed by a meteor impact 700 ± 5 million years ago placing it in the Cryogenian period. The crater is 14 km in diameter.
Prior to the Second World War, the lake was thought to be the second known volcanic caldera in Finland (the other was Lake Lappajärvi). Both were eventually recognized as impact craters.