all my life i have had a thick-soled foot. if my feet get damp for a day or two, small holes the size of a pencil lead appear and deepen, sometimes filled with whatever else is in my shoe (like socklint or if i am hiking, dirt). the longer it goes the more holes till the hole sole is covered. i dont ever let it go that far. i keep it in check by changing socks often, NEVER re-wearing a pair without washing, and gold bond powder. it is not painfull and whatever is eating the skin stops at the live tissue underneath. its just very creepy looking.
never seen anything like it and havent found anything to make it go away. its just a cosmetic thing so i have not seen anyone about it. email me if you want to see a pic.