Abstract: A modular caisson for use in performing underwater tasks in a dry environment includes a plurality of interconnected modules. The end of at least one of the modules is equipped with an end cover through which piping or other equipment can be passed into the caisson without leakage of water into the caisson. In a preferred embodiment, the end cover includes flexible doors. The caisson can be supported by a transport vehicle, or it can be anchored to piles by means of guides formed on the outside of the modules. As a result, the caisson can be prevented from exerting undesirable loads on the piping or other structure being worked upon inside the caisson.