As Harry falls into the tunnel, he'll land directly into a huge pit of mud that seems to be occupied by a large number of Paldean and regular Quagsire. It's impossible to move in any direction except through forceful wiggling (which only results in one sinking deeper into the mud), which means that Harry might be stuck for a while if he doesn't get help. Not to mention the amount of laundry that he's going to have to do once he gets out.

Hey, at least he has company. The Quagsires are happy to share the mud pit with him.