The Cave Diving Group (CDG) is a United Kingdom-based diver training organisation specialising in cave diving. The CDG was founded in 1946 by Graham Balcombe, making it the world's oldest continuing diving club. Graham Balcombe and Jack Sheppard pioneered cave diving in the late 1930s, notably at Wookey Hole in Somerset. Passages through caves are often blocked by a submerged section, or sump. Cavers in many countries have tried to pass these barriers in a variety of ways; using the simple "free dive" with a lungful of air or by utilising the available diving technology of the day. Early history of cave diving in the UK Two Post Office engineers, (Francis) Graham Balcombe and (John Arthur) "Jack" Sheppard, who were among the leading climbers and cavers of their era, combined their energies into solving the problem of passing the Swildon's sump. Their pioneering dive on 17 February 1934 used a home-made respirator, designed by Balcombe, that incorporated part of a ladies' bicycle frame. The attempt was unsuccessful but the foundation of cave diving in the U.K. had been laid. In 1935, Balcombe made enquiries with Siebe Gorman, the leading diving engineers of the time. Although they could not help with light weight diving equipment, they did offer training and the loan of standard helmet diving gear. A successful expedition was mounted at Wookey Hole, where the waters from Swildon's Hole resurge. Exploration of the subterranean River Axe proceeded to the seventh chamber, a distance of , which was as far as Balcombe and Diver No. 2, Penelope Powell could physically handle their pipes and ropes. The team returned to Swildon's Hole on 4 October 1936 with Jack Sheppard's newly designed apparatus nicknamed "Jimmy". It was fed by a football inflation pump that was coupled to a home-made drysuit that incorporated lighting and a telephone. This was used by Sheppard to pass Sump l. Later that year the bicycle respirator became self-contained with the addition of an oxygen cylinder. In turn Balcombe dived solo through Sump 1 and found the air chambers of Sump 2. These pioneers established the pattern of UK cave diving; the systematic exploration of a cave system in its phreatic zone by diving both the resurgence and its tributaries. During the war years Balcombe developed a self-contained closed-circuit oxygen set using mainly medical components. In the Yorkshire Dales he began the great canon of exploration, now known