Ahhh! Good to see this picture as the Twll was still filling up. I was shown around Dorothea in it's last year of operation by the then manager (can't remember his name after all these years). Going down to the bottom in a large Cat dump wagon to collect slate blocks was an experience - through a tunnel at the bottom of the pinnacles! Beneath the Blondin tower is a gallery with an abandoned Ruston Bucyrus digger on it. I sat in the cab and played with the levers. In about 1975, I again sat in the cab playing with the levers but was wearing scuba gear and had about 50ft of water over my head...
The top of the pinnacles is what causes all the diving deaths IMHO - unskilled divers have heard about it and go looking. As you see in the picture, the top is very small in cross section. Unfortunately, the top is currently about 90ft beneath the surface and this is just about where the 'narcs' hits them (Nitrogen narcosis - it's just like you had 10 pints of lager but the effects come on instantaneously). If you've not been trained in deep diving and the effects the narcs has, then it's bye-bye... They get disorientated and then sink. It's over another 300ft to the bottom and that's that...
BFN
Clive the (ng railway) Bogieman