At the risk of hijacking this thread....
Spider holes fascinate me.
They do not always occur in the same parts of lake each year, sometimes they stay open or grow in width, and sometimes they freeze over for the rest of the winter. As ANDN implies, the ice a meter to the side of a spider hole can be a full winter's depth - 2 or 3 feet.
While watching them when they are open, it is easy to see that they are the "vent" for liquid water to flow up or down from the top of the ice to the bottom & vis-versa, perhaps b/c of atmospheric pressure changes or b/c of changing snow loads upon the ice. Springtime melt water trapped on top of the ice "escapes" back into the liquid lake down though these spider holes for sure - sometimes with a small whirlpool on the surface.
Is anyone else interested spider holes? What do you think causes those "spider holes" initially, and why do they happen where they happen?