The Vikings' eastward expeditions took them through the Gulf of Finland and on to Lake Ladoga. Over the centuries, fierce sea battles have been fought for control of that vital trade artery the Baltic. One indication of its continuing importance is Russia's determination to hang on to the Kaliningrad region, a detached sliver of territory between Poland and Lithuania that bristles with air and naval bases. Small wonder, therefore, that the waters of the archipelagos fringing the Finnish coast contain so many relics of history waiting for researchers to find them - if only they can. The low salinity means that the wood-eating shipworms common in the oceans do not thrive here.
Wooden wrecks are very well-preserved.
On the other hand, ice can be a mighty destructive force. When it begins breaking up in spring, formations of pack ice can reach as far as twenty metres below the surface of the sea and crush all beneath them. And naturally, the incessantly moving sea shifts everything lying on its bed. If a wreck lies beneath the channels that large vessels ply, it can likewise suffer damage. For example, a warship sunk off Kotka during the Battle of Ruotsinsalmi in 1790 happens to lie below a turn in the channel, a place where ships' propellers are revved up.