Which country's fjord Sognefjorden is Europe's longest?

Norway, and Sognefjorden runs 205 kilometres inland and over 1,300 metres deep - deeper than the sea outside it, which is the defining oddity of a fjord. Glaciers carve below sea level because ice is not water and does not stop at the shoreline, so a fjord is typically deepest in the middle and has a shallow sill at its mouth.

That sill matters biologically. It restricts exchange with the open sea, so deep fjord water can stagnate and go anoxic, and some Norwegian fjords have layers at depth that have not mixed in centuries. It also makes them superb natural harbours - deep, sheltered, and hard to attack, which is why the German navy used them in the Second World War and why the Tirpitz spent much of the war hiding in one.

Norway's coastline is the standard example of a measurement problem. Counting the mainland as a smooth line gives about 2,500 kilometres; counting every fjord and island gives over 100,000, which would put a country of five million people among the longest coastlines on Earth. The figure depends entirely on the resolution you measure at, which is the coastline paradox, and it is why national coastline statistics should be treated as descriptions of methodology rather than of geography.