It was an accident that occured in the Porth-Yr-Ogof resurgence- a notriously dangerous way of exiting the cave. It's tempting to swim it, but apparently there are barely submerged, sloping ceilings which extend up to 10 meteres across the pool away from the airspace- like a big triangular passage. So there is a risk of being swept underneath one of the sloping walls and drowning- apparently there have been 10 seperate incidents previously.