|Diameter||9 km (5.6 mi)|
|Municipality||Sund, Lumparland, Lemland & Jomala|
Location of the crater in the Botnian Gulf
Most of the bay fills a nine kilometer wide impact crater. The crater is estimated to be about 1000 million years old (Proterozoic). The depression was originally believed to be a rift. Extraterrestrial origin was first proposed in 1979, but not until 1993 was the impact structure finally confirmed. Long shatter cones have been discovered in the southwestern part of the bay. The crater is filled with sediments, between Pleistocene sediments and crushed rapakivi granite bedrock there is a layer of Paleozoic (Ordovician) limestones. This makes Lumparn one of the few places in Finland where fossils have been found.