when Gordon Marriott lost his life returning from the ninth chamber. Another fatality occurred in 1981 when Keith Potter was drowned on a routine dive further upstream. Further progress required apparatus that could overcome the depth limitation of breathing pure oxygen. In 1955, using an aqualung and swimming with fins, Bob Davies reached the bottom of the 11th chamber at depth in clear water and discovered the 12th and 13th chambers. He got separated from his guideline and the other two divers in the 11th chamber, ending up spending three hours trapped in the 13th chamber, and had much trouble getting back to safety. Opinion hardened against the use of the short-duration aqualung in favour of longer-duration closed-circuit equipment. Likewise, the traditional approach of walking along the bottom was preferred over swimming. Employing semi-closed circuit nitrogen-oxygen rebreathers, between 1957 and 1960 John Buxton and Oliver Wells went on to reach the elbow of the sump upstream from the ninth chamber at a depth of . This was at a point known as "The Slot", the way on being too deep for the gas mixture they were breathing. A six-year hiatus ensued while open circuit air diving became established, along with free-swimming and the use of neoprene wetsuits. The new generation of cave diver was now more mobile above and under water and able to dive deeper. Using this approach, Dave Savage was able to reach air surface in the 18th chamber (chambers did not have to have air spaces to be so named; they were the limits of each exploration) in May 1966. A brief lull in exploration occurred while the mess of guidelines laid from the ninth chamber was sorted out before John Parker progressed first to the large, dry, inlet passage of the 20th chamber, and thence followed the River Axe upstream on a dive covering at a maximum depth of to the 22nd chamber where the way on appeared to be lost. Meanwhile, climbing operations in the ninth chamber found an abandoned outlet passage that terminated very close to the surface, as well as a dry overland route downstream through the higher levels of the eighth, seventh and sixth chambers as far as the fifth chamber. These discoveries were used to enable the show cave to be extended into the ninth chamber and the cave divers to start directly from here, bypassing the dive from the third