‘It’s an incredible buzz to explore further than anyone has been ever before,’ said Mr Mallinson, from Huddersfield. ‘There was no wildlife down there, just a tunnel of crystal blue clear water stretching on and on.’
Mr Mallinson, 43, added: ‘The adrenaline builds when you realise you are looking at something nobody has ever seen before. It’s that which drives you forward.
‘You don’t get scared. But you are permanently conscious of your equipment. If the slightest thing goes wrong then you are in a position where you might never be coming back.’
After sump two they emerged in a small dry cave area nicknamed Tipperary. It was there they spent two nights resting and replenishing their underwater breathing mixtures.
They beat the 7.8km (4.8-mile) world record for the longest cave dive penetration, set last year at Wakulla Springs in Florida.
Support diver Martyn Farr, 59, said: ‘This exploration is akin to the first conquest of Mount Everest in 1953.’