Underwater construction requires specialized equipment, training, and safety measures. Divers perform inspection, demolition, installation, and repair work underwater. Commercial diving training and certification are required. Decompression requirements depend on depth and bottom time. Air mixtures including trimix reduce nitrogen narcosis at depth. Surface-supplied diving provides advantages over scuba at moderate depths. Umbilicals supply air, communication, and life support. Saturation diving allows deep work by extending bottom time at pressure. Divers remain at pressure in habitats rather than decompressing repeatedly. Deep saturation dives are expensive but allow significant work time. Construction activities underwater include pile driving for marine structures, cable installation, and maintenance of submerged equipment. Underwater welding requires specialized training. Explosive charges may be used for demolition underwater. Sonar and submersible equipment provide inspection of deep underwater structures. Underwater concrete placement requires special procedures and materials. Remotely operated vehicles (ROVs) perform tasks at extreme depths avoiding diver risk.