bark peeling all the way down to cambium on London Planetree

I have a 25 foot tall London Planetree in the back yard.
This spring the bark separated from the wood at the base of the trunk, to a height of about 18 inches, almost all the way around the circumference. (This is _not_ the normal bark peeling one would expect for this tree).
The bark peeling off is about 3/8 inch thick, and bare wood (sapwood?) is exposed almost all the way around the trunk near the ground.
When the bark first started falling off, about 4 weeks ago, the color of the exposed wood was orange-brown. Now the color has darked considerably to a brownish-black.
Just today, I noticed there are about 2 dozen tiny holes in the bare wood, about the diameter of a pin, and I saw something moving in a couple of them. (When the bark first started falling off, about 4 weeks ago, there were no such holes).
Surprisingly, there are buds on the tree and leaves appear to be forming normally.
This was one of my favorite trees, and I have been watching it grow vigorously for 5 years.
I've done a bunch of googling, and talked to several "master gardeners", but nobody has a clue what is going on, or why, or if the tree is about to die, or if there is anything I can do to save it.
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