Underwater construction is industrial construction in an underwater environment. It is a part of the marine construction industry.[1] It can involve the use of a variety of building materials, mainly concrete and steel. There is often, but not necessarily, a significant component of commercial diving involved.[2][3] Some underwater work can be done by divers, but they are limited by depth and site conditions, and it is hazardous work, with expensive risk reduction and mitigation, and a limited range of suitable equipment. Remotely operated underwater vehicles are an alternative for some classes of work, but are also limited and expensive. When reasonably practicable, the bulk of the work is done out of the water, with underwater work restricted to installation, modification and repair, and inspection.