as the Three Counties System with dives at Keld Head, Goyden Pot and Alum Pot. Formation of the Cave Diving Group After World War II surplus oxygen rebreather equipment became available, and the number of divers increased. In 1946 these enthusiasts had a meet in South Wales during which they formed the Cave Diving Group. Wookey Hole once more became the focus of diving activities. In addition to exploration, the early divers found a lot of archaeological material. As the divers penetrated deeper into the cave, the need for penetration below (the safe limit of oxygen diving) became more urgent, and the first attempt with an early aqualung nearly ended in tragedy. Following this incident, the group concentrated on the use of semi-closed circuit rebreathers with oxygen/nitrogen mixtures (nitrox) suitable for the depth. In 1960 the way forward at Wookey Hole was found at a depth of , using 60/40 O2/N2. Rebreathers were also used successfully in South Wales, Derbyshire and Yorkshire. Accomplishments During the 60s, with the abandonment of National Service, there was an increase in caving activity by the post-war population bulge. Readily available commercial scuba equipment was adapted to cave diving. The process was accelerated by the Cave Diving Group's publication of "Cave Diving on Air" by Mike Boon and the formation of the Independent Cave Diving Group. Divers began to dive deeper and further, and inevitably there were tragedies. Two innovations were introduced in the interest of safety. Separate cylinders, each one with an independent demand valve; and the adoption of the "Rule of Thirds": one third in, one third out, with one third kept as a safety reserve. In the early days of cave diving in the UK there was always the hope of finding a way into the cave for the benefit of dry cavers. Amongst these must be mentioned Swildon's Hole, Stoke Lane Slocker, Ogof Ffynnon Ddu II and Little Neath River Cave. Progress and development of equipment and techniques have always come in phases, and with this progress, although still hoping to discover dry caves, came diving of a purely exploratory nature. This has led to advances at Wookey Hole and Cheddar Gorge caves in Somerset. In Derbyshire a great deal of effort has produced significant discoveries in the Peak Cavern/Speedwell Cavern system. In South Wales many kilometres of Daren Cilau were first trodden by cave divers; and the link with Elm