way for the exploration of the Gouffre Berger, the first cave in the world to break the 1 km depth limit. Early systems of ascending ropes were developed by Pierre Chevalier in the Dent de Crolles cave system in France in the late 1930s, Chevalier also being the first to use nylon rope in a cave as opposed to natural fibre rope. Single rope technique (SRT) began to be developed in the US in the 1950s. A similar system was developed in Europe in the late 1960s, which was quickly standardised and is still in use today. SRT offered the advantage of greater speed and versatility in the descent of vertical shafts. (Previously, one caver would have to remain at the head of the final pitch to belay the returning cavers up a ladder climb.) The increasing popularity of caving during the 1960s and 1970s led to the creation of specialist caving equipment companies, such as Petzl. Previously, cavers adapted equipment from other sources, such as miners' helmets and electric lamps, or made their own equipment. Caving equipment made today conforms to high safety standards, decreasing injuries and fatalities. Protective clothing Thermal protection Caves in temperate regions such as Europe and North America maintain an average yearly temperature of 11–13 °C (52–58 °F). While this is not especially cold, exposure to water and fatigue can increase the risk of hypothermia. Cavers usually wear a one-piece undersuit made of fleece or fibre pile, sometimes used in tandem with thermal underclothes. In warmer caves, such as those in France and Spain, lighter undersuits are used to prevent overheating. When caving in wet caves neoprene wetsuits provide superior insulation to fleece underclothes. While cavers often use wetsuits designed for surfing or diving, specialist caving wetsuits are available with reinforced elbows and knees. Hybrid fleece wetsuit-undersuits are also used. Abrasive protection Cavers commonly wear protective oversuits, similar to boiler suits but made of heavily abrasion-resistant material such as cordura. In wet or windy caves, PVC oversuits may be preferred, as they provide a greater degree of warmth and protection against getting wet. Oversuits often come with reinforced areas, especially at wear points such as the elbows, seat and shins. Internal pockets and hoods are sometimes provided. Knee pads and, less commonly, elbow pads are worn both to protect both the caver's person and clothing. Gloves are also worn. In wet caves neoprene gloves can