Jay,
It was weird.And some time ago (early 80's), so my details may be slim- but it is not easily forgotten.We were somewhere in NJ- I think south of Barnegat( I wasn't driving the car or the boat, so who paid attention.)
The tunnel was not deep (40-50 ') nor was it very large -the "local" diver ( he lived in the area and owned the boat we used) estimated it was approx 75-100 ft in length(I thought it was shorter),in "length"with an average "width"of 3-4 '(about right)-sort of a very long swim through.
Anyway, 3 of us searched the area in and around the tunnel, looking for ???, while the other 2 returned to the boat. While they brought the boat back to where we were, the "boat sitter" radio'd it in .
When some of our air supplies got low, we surfaced, boarded the boat, and stayed there for about an hour. As it was getting to be dusk, we dove back with the two least empty tanks, and retrieved the gear.
The guy who owned the boat took the gear to the police,along with the info as to what happened.
Perhaps most creepily, that was the end of the story. Nobody ever reported a loss, and nobody was reported missing-weird.
A month or so later, my friend with the boat claimed the gear- but never used it. I don't blame him.
As I said, weird!!,
Mike