In the 1930s, as caving became increasingly popular in France, several clubs in the Alps made vertical cave exploration an outdoor sport. During World War II, a team composed of Pierre Chevalier, Fernand Petzl, Charles Petit-Didier and others explored the Dent de Crolles cave system near Grenoble, France, which became the deepest explored cave in the world (-658m) at that time. The lack of available equipment during the war forced Pierre Chevalier and the rest of the team to develop their own equipment, leading to technical innovation. The first use of single-rope technique with prusik and mechanical rope ascenders (Henri Brenot's monkeys, first used by Chevalier and Brenot in a cave in 1934) can be directly traced back to the exploration of the Dent de Crolles cave system. American caver Bill Cuddington, known as Vertical Bill developed single-rope techniques in the U.S. in the late 1950s. In 1958, two Swiss alpinists, Juesi and Marti teamed together, creating the first commercially available rope ascender known as the Jumar. In 1968 Bruno Dressler asked Petzl, who worked as a metals machinist, to build a rope-ascending tool, today known as the Petzl Croll, that he had developed by adapting the Jumar for pit caving. Petzl started a small caving equipment manufacturing company Petzl, which manufactures equipment for caving, climbing, mountaineering and at-height safety in civil engineering. The rappel rack was developed in the late 1960s by cavers in the Huntsville, Alabama caving club to facilitate long descents. The evolution of mechanical ascension systems helped extend the practice and safety of pit exploration.