my guess is its getting very hot in that cyl. only guess with TBI would be that the cyl isnt being lubricated completely, making extra heat, explosions are stronger, and melted the ground?
I'm probably way wrong, but it's an idea, a possibility. Seems logical, but out there.
Pull the head, and check what the cyl walls and the head looks like. also, maybe an ignition problem, for some reason getting stronger sparks, acting like a plasma cutter?