Commercial divers build, repair, and inspect structures that are underwater. Divers may weld underwater cracks in offshore oil and gas rigs, lay the foundation for a bridge piling, inspect pipes in water treatment plants, or rig and remove a 300,000-pound piece of concrete from a river bottom. Commercial diving is hard physical work; it is critical that commercial divers be able to adapt quickly under adverse conditions and work efficiently. Depending on the location or type of project, commercial divers may work under difficult conditions including tight spaces, low visibility or long durations in cold water. Some commercial divers may travel extensively based on where their expertise is needed.