Nøklevann in Oslo on a foggy late winter/early spring day, before ice has melted. The first part of the name ("Nøkle") is derived (and somewhat distorted) from the norse word mikil which means great.
So the name simply means the great lake. Although it takes some time to walk around it, it isn't *that* great compared to other great lakes in Norway. On a day like this, however, the fog really gives the impression that the lake continues endlessly and that it deservedly bears its name.