All of a sudden.  Just suddenly started running 10 degrees hotter with the fan going 100%.  Fans are fine, I replaced the original sleeve bearings with sealed ball bearings years ago and they been fine ever since.  For sure, the crater wasn't there when I changed the fans so it has happened while it's been running.  It's like a teeny weeny explosion, damaging the GPU and heatsink.  But it still runs fine.  I will get some fine emery paper and see if I can smooth it down a bit.