 In the 1930s, as caving became increasingly popular in France, several clubs in the Alps developed vertical cave exploration into a recognized outdoor sport.During World War II, a team composed by Pierre Chevalier, Fernand Petzl, Charles Petit-Didier and others explored the Dent de Crolles cave system near Grenoble, France.It became known as the deepest cave in the world () at that time.