Cofferdams are temporary structures built around construction sites located underwater. They allow construction to take place in a dry environment by sealing off the work area from water. There are several types of cofferdams including earthen, single-walled, double-walled, cellular, and rock-filled varieties. Construction typically involves driving sheet piles into the ground to form the cofferdam walls, placing a concrete seal, and then pumping water out of the enclosed area. Cofferdams provide a safe, dry work area and allow re-use of construction materials, though they present hazards like falls and contaminated water.