Mystery: Is The Tree OK?

Found these holes around the sycamore tree on the corner lot the other day. They are all approximately three feet out from the trunk. There are 31 of them, 33 if you count two that are smaller than the rest. Fairly evenly spaced. They go completely around the tree.

The holes are anywhere from four inches to ten inches deep. Majority measure between six and eight.
(Note to the Dark Smurf: No jokes about this please!)
So anyway.... Back to the mystery.  I have no idea what made these. There was a toad in one of them as I was measuring depth. Surprised the poor little thing so bad! But they are opportunists as far as finding a place to stay. So I don't think that toads would be creating something this symetrical. The openings are a little less than an inch in 'width' and a little more than in 'height'.
Any ideas????????
My main concern is that the tree is safe and sound. Any info would be greatly appreciated.