Iso-Naakkima is a lake in Southern Savonia, Finland, about 10 km south of the town of Pieksämäki. It is notable having an eroded Impact crater under the northwestern part of the lake. The crater is one of the oldest known, about 1200 million years old (900–1200 Ma, Mesoproterozoic). At the time of the impact event the continent of Baltica (modern northeastern Europe) was located near the equator. The crater is filled by recent (Quaternary) sediments and is not visible to the surface. Its existence was revealed by strikingly circular geophysical anomalies which are concentrated in a region 3 km in diameter; the actual remnant is about 2 km wide and 160 m deep. Its existence was later confirmed by drilling results.