you found one of the few scenarios that actually effectively game-overs you (digging it up and not keeping an eye on it)
you can try to get busy smashing it with a crowbar/knife... or maybe the pickaxe is effective against it, but I think that's too much at this point, you've doomed the world kel
object and event reset might remove it but I've not tested it, your choice if you want to persist in your doomed save or try to fix it. If you do an object and event reset, make sure you put all the items you care about into the mailbox and save first.