[cripplecreek]

Here's one on Mud lake in Liberty Township Michigan.



If I recall correctly they form pretty much every year.

[stayathomemom]

I know nothing about this so I'll opine.;-) I would think that the presence of underwater springs in a spring fed lake could cause a uniform thawing of ice in a circular shape. If the temperature then dipped low enough, this area could refreeze. Why it would apparently freeze from the center out, I don't know, since I know nothing about this anyway.;-)