What is hell diving?

I think you mean BELL diving, it is diving down in a Diving bell

There is a term I have heard of by that name. As far as I remember, "Hell Diving" is the practice of spear fishing for sturgeon beneath the oil derricks in the Gulf of Mexico off the coast of Louisiana. It is apparently pretty dangerous for a number of reasons including depth (60m), visibility, size of prey (Sturgeon are kind of big) etc... Not sure if this is what you were referring to but to the best of my recollection that's what it is. Cheers