Nog een nogal bederfelijke vraag over Doctor Who S9E11 Heaven Sent .
When he first falls into the water, he sees thousands of skulls lying on the bottom of the sea.
Later on, when we find out that he's been here and has died many times before, it's very strongly hinted that those are all his own skulls: when he dies, his skull is all that remains and it falls in the water later.
So... were those all his skulls from his previous incarnations? If so, why weren't there billions of skulls in the water near the end? If not, do we have any indication whose skulls they were? And/or, was the situation on the seabed also "reset to its initial state" regularly?