Thousands of years ago, the Saimaa seals were ordinary sea seals living in the Baltic Sea. Then the ice from the last Ice Age melted and the land slowly lifted up, cutting Lake Saimaa off from the sea. The seals were trapped. Over generations, they got used to fresh water and became their own special kind.
A Saimaa ringed seal has dark grey fur covered in little pale rings - that's where it gets its name. Adults are around 1.5 metres long and weigh about 80 kilograms. They are very shy. Most Finns who live near the lake have spent their whole lives there without ever seeing one.
Mother seals give birth in February, inside a snow lair on a frozen patch of the lake. The pup is born with white fluffy fur to keep it warm and hidden. For a few weeks, the mother visits to feed the pup, then the pup slips into the water and starts learning to swim.
There are not many of these seals left. Climate change is a big worry - in winters when the lake doesn't freeze enough, the mothers can't build their snow lairs and the pups are in danger. Lots of Finnish children take part in 'snow lair' projects: in winter, volunteers actually push up little piles of snow on the frozen lake to help the seals out.