I've talked to a couple of friends at the Electric Boat Yards
about this.  According to them, they are susposedly testing a DSRV
and using the drilling as a cover.
But they did mention something to the effect that the Mariana's
is the only place left on earth where the US can test nuclear
toys and not be detected by other governments.

I'm still nosing around for my own sake of curiosity on this.
Since the Russians have been using the Mariana's for scuttling
their own nuclear ships that have either gotten too irradiated
because of their reactor designs, or outdated in general,
I'm wondering if what is happening out there isn't nothing more
than a covert salvage operation.    After all, "We" wouldn't want
to be actually caught with another countrys' toys in our toybox
would we ? 

------------------------------------------------------------ 

Aye, and what brand of nuclear toy?  The old crude concussion 
gizmos don't seem to be all that fashionable lately.  Everybody 
and their dog seems to be going pell-mell after the pulse.

Maybe they are as in Tesla-lore "banging in rhythm on the water pipes."