Underwater welding refers to a number of distinct welding processes that are performed underwater. The two main categories of underwater welding techniques are wet underwater welding and dry underwater welding, both are classified as hyperbaric welding. In wet underwater welding, a variation of shielded metal arc welding is commonly used, employing a waterproof electrode. Other processes that are used include flux-cored arc welding and friction welding. In each of these cases, the welding power supply is connected to the welding equipment through cables and hoses. The process is generally limited to low carbon equivalent steels, especially at greater depths, because of hydrogen-caused cracking. In dry underwater welding the weld is performed at the prevailing pressure in a chamber filled with a gas mixture sealed around the structure being welded. For this process, gas tungsten arc welding is often used, and the resulting welds are of high integrity. The applications of underwater welding are diverse—it is often used to repair ships, offshore oil platforms, and pipelines. Steel is the most common material welded. For deepwater welds and other applications where high strength is necessary, dry underwater welding is most commonly used. Research into using dry underwater welding at depths of up to 1000 m are ongoing. In general, assuring the integrity of underwater welds can be difficult (but is possible using various non-destructive testing applications), especially for wet underwater welds, because defects are difficult to detect if the defects are beneath the surface of the weld.