(Takes name from Kalevala, of course... the inspiring mythological entity should be noded later, by me or by someone else.)
Ilmarinen was an armored ship. Its construction was commissioned by the Finnish Naval Forces in 1928, and it was built on Crichton-Vulcan shipyard in Turku. Two such ships were built there; the sister ship was called Väinämöinen. Väinämöinen was finished in 1932 and Ilmarinen year later.
Strategically speaking, the ships were mostly designed to guard the large archipelago, particularly Åland. They operated on Saaristomeri and Ahvenanmeri areas. During the Finnish Winter War Ilmarinen defended the city of Turku as a supplement to the air defences, and during the Continuation War she bombarded the USSR-occupied city of Hanko.
Details
Ilmarinen was, without question, the flagship of the Finnish navy. She remains, to date, the biggest warship built in Finland. Some strategic measurements:
Armament:
In addition to this, the ship was equipped with state-of-the-art mechanical cannon fire target calculation system and electronic signalling and fire command equipment.
The fateful day...
Ilmarinen sank September 13, 1941. The ship was on a decoy mission (Operation Nordwind) to divert the Red Army's attention a bit while the Germans tried to invade Estonia. The mission was a failure - the USSR forces didn't even see the convoy, and Ilmarinen hit a sea mine.
The ship had two paravanes (submerged thingies attached to sea-mine-anchor-chain-cutter wires) on the sides. 25 nautical miles south of Utö, one of the ship's officers, captain lieutenant Ilmari Huhta noted that one of the paravane wires was down to the side of the ship - and suspected (correctly) that the paravane was dragging a mine of which it couldn't cut the anchor chain. The ship stopped, the wire was pulled but it didn't move, so they had no choice but to continue the journey. When the ship turned starboard sharply, the mine that was being dragged by the paravane got detached, hit the ship and was set off. Some claim there were two explosions, so it is possible there were two mines stuck on the wire.
The ship sunk in seven minutes - it first turned upside down, stayed afloat for four minutes, and went down like a rock. The convoy's four nearest patrol boats were able to pick up 132 survivors, but 271 men were lost, not being able to abandon the ship (very few of the people below deck or in the artillery turrets managed to escape - it is not known how many were trapped and how many drowned while escaping).
The memorial service was held on Väinämöinen on September 21, 1941, and the loss of the ship was officially announced the next day.
The wreckage was found in 1990. Ilmarinen rests upside down mostly buried in seabed, in depth of 70 meters, south of Utö. As an intesting coincidence, the wreck of passenger ship Estonia lies only 15 kilometers away.
Sources:
http://koti.mbnet.fi/~karsikas/psl/
http://www.hut.fi/~jaromaa/Navygallery/Coastal/Nordwind.htm
http://www.nba.fi/MUSEUMS/MARITIME/julkaisu/ilmari.htm