be worn as added protection against the cold. Footwear Wellington boots are a popular choice of footwear, being hard-wearing, cheap, having good grip and great water-resistance. Hiking boots are also worn, providing superior ankle support. They do however let water and grit in much easier and are often damaged by the harsh cave environment. There is also the risk of lace hooks ensnaring on ladders. In large, dry, tropical caves they are superior to Wellington boots, being cooler and restrict movement less. Specialist canyoning boots offer an expensive alternative to Wellingtons and hiking boots. Helmets While helmets are used to protect the caver's head against occasional falling rocks, they find much more use in protecting the cavers' heads from bumps and scrapes while moving through low or awkward passageways. Helmets are invaluable for mounting lights—an array of lights can be attached to a helmet. Many helmets used in caving can also be used as climbing helmets. Vertical equipment Many caves have shafts or drops that require ropes or ladders to pass. Wire ladders have largely been superseded by ropes for descending Pitch (vertical space) since the early '60s, though ladders still have useful applications on shorter pitches, where full abseiling gear would be inappropriate. Single rope technique The single-rope technique (SRT) is the most commonly used technique for passing vertical obstacles. Standard equipment Climbing harness - static and more abrasion-resistant than the harnesses used in rock climbing. Climbing rope - rope used for abseiling is low-stretch static rope, typically 9 mm-thick in Europe. In the US, SRT rope is thicker (11mm) and more abrasion-resistant, given the greater amount of rope-rub tolerated. Ropes are cut into varying lengths Ascender - used to ascend ropes. Devices that used lever cams were once used, though these have now been overtaken in popularity by toothed cam devices which slip less. A minimum of two ascenders are used, one attached to the harness at waist level, and another attached to a foot-loop and moved by hand. A third ascender may be attached to a foot and a rope-walking technique used. Descender - used to abseil down the rope. There are two main types of descender – a bobbin descender such as the Petzl Stop, and a rack descender, favoured in parts of the US for its smooth descent and excellent heat-sinking capacities. Bobbin descenders are favoured for European style SRT as they make it easier