I am wondering if crabs always refill the holes they dig, or if something terrible has happened and one of the other crabs filled it in for some reason? And now he's going to suffocate all alone in his underground lair?

Common sense would tell me that they need their caves open in order to get air while they're down there, but I'm no expert. We have a 29 gallon aquarium with five small PPs and 6-7 inches of play sand/eco earth 5:1 mix for substrate, if that matters. Nice and moist with sand castle consistency, as everyone seems to recommend.