Water is most dense around 4°C. Accordingly the bottom of a large body like the great lakes is near that temperature year round. This is part of the reason why fish can survive in lakes that freeze over in the winter.

This is exploited in deep water source cooling [1].

Such a thing is implemented in Lake Ontario since 2004.

Very cool imo :)

[1] https://en.m.wikipedia.org/wiki/Deep_water_source_cooling