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.