People, not sane people mind you, but they are still considered people, have been reaching into holes for a long time to pull out catfish. They can also pull out snakes and other non-wonderful things... thus the "not sane people" part. People now set boxes underwater for the catfish to use (like nest boxes) and then go in, stick their hands in the box and pull out a large catfish... or a small catfish... or a small gator... or a nub and something had hand for breakfast.
Some catfish in places where there is a wet season and a dry season wiggle their tails into holes so they can stay in the water or at least damp. The catfish who do this are among the dual breathers, they have gills but also have a primitive lung-like thing that they can breathe air through.
Filling up the hole with something putting off that much carbon dioxide would make both of their breathing systems malfunction and they are probably coming to the surface to gulp air. ? . It's a theory at least.