Some interesting facts from those in the know on both the sub and how to find it on the Jeremy Vine radio show just now.
Some bloke who has done the trip in it explained that once they hit bottom they knew they were within 500metres of essentially a giant scrap yard and steel debris field, but they had no way of knowing how close or in which direction due to the pitch darkness there. He described it as just scrabbling around on the ocean floor for an hour and half looking for something.
Sounds fucking ludicrous, and predictably lethal.
If stuck on the seabed they are stuck waiting for a rescue that is virtually impossible. If they have popped up its a needle in a haystack and then it needs conditions to be such that someone can be physically able to undo the bolts that seal it.