Further progress required apparatus which 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.