Spring holes occur over springs or underwater culvert outlets that bring water warmed by the its passage underground. They form when the inflow of heat from the spring is not overpowered by the rate of freezing of the ice. Often in mid winter, ground water flows diminish which also lets them freeze over easily. They often re-form in the spring after as the weather warms up and ground water flows increase. They occur at the same place from one season or year to the next (gas holes and reef holes also have this characteristic).
After a warm spell in early spring, the hole shown below opened up. I dangled a thermister into it and found changes in temperature of a few degrees over periods of roughly 20 seconds. I expect this is from the turbulent flow of low density warm water up through the colder pond water. For this to work the incoming water needs to be warm enough and have enough flow to penetrate the the reverse density layer that is often at least several feet thick under the ice sheet. I did not see these temperatgure swings when I did the same thing in mid winter when the hole was covered with relatively thick ice and the culvert flow was probably quite low.
While many people attribute holes to springs, spring holes are much less common than you might think. Most of the time they are gas holes which can easily be identified by watching them for a few minutes. If any bubbles come up it is a gas hole. New-ice holes also get blamed on springs but they are only found on new ice and they do not occur at the same place from year to year. I have looked at a lot of holes and the only two I was reasonably convinced were spring holes are show in in the adjoining pictures.
Bob May 2012