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It's a dirt hole. My guess is that you create a large amount of mud at the bottom of the hole and then the walls collapse.

Presumably warm extremely-salty water, or it might get hypothermic.

However, if it's somehow wedged in there, the dog could drown.

I think you need to get a camera down there to monitor the progress of the solution

Or sand

or mythbusters style fill with ping pong balls. Probably both bad ideas.