An underwater body (101, 201, 301) having - a shell (103, 203, 303), - a movable component (107, 225, 333), - an expansion means (111, 113, 115, 117, 119, 213, 215, 231, 313, 315, 331) and - a hollow space, wherein the movable component (107, 225, 333) is movable - relative to the shell (103, 203, 303) from a retracted into an extended position and - is operatively connected to the hollow space, wherein the volume of the underwater body (101, 201, 301) with the movable component (107, 225, 333) in the extended position is greater compared to the volume which the underwater body (101, 201, 301) comprises with the movable component (107, 225, 333) in the retracted position, and wherein the expansion means (111, 113, 115, 117, 119, 213, 215, 231, 313, 315, 331) is designed for the purpose of - conducting a fluid (121, 221, 321) into the hollow space and, - as a result, of moving the movable component (107, 225, 333) into the extended position, wherein the underwater body (101, 201, 301) is designed such that - the fluid (121, 221, 321) in the hollow space hardens and - the hardened fluid (121, 221, 321) holds the movable component (107, 225, 333) in the hollow space in the extended position;. characterized in that the hollow space is connected to the movable component (107, 225, 333) via a piston cylinder unit and the hollow space is formed in a chamber of the piston cylinder unit such that the expansion means conducts the liquid fluid into the chamber as to displace a piston of the piston-cylinder unit. - a shell (103, 203, 303), - a movable component (107, 225, 333), - an expansion means (111, 113, 115, 117, 119, 213, 215, 231, 313, 315, 331) and - a hollow space, wherein the movable component (107, 225, 333) is movable - relative to the shell (103, 203, 303) from a retracted into an extended position and - is operatively connected to the hollow space, - relative to the shell (103, 203, 303) from a retracted into an extended position and - is operatively connected to the hollow space, wherein the volume of the underwater body (101, 201, 301) with the movable component (107, 225, 333) in the extended position is greater compared to the volume which the underwater body (101, 201, 301) comprises with the movable component (107, 225, 333) in the retracted position, and wherein the expansion means (111, 113, 115, 117, 119, 213, 215, 231, 313, 315, 331) is designed for the purpose of - conducting a fluid (121, 221, 321) into the hollow space and, - as a result, of moving the movable component (107, 225, 333) into the extended position, - conducting a fluid (121, 221, 321) into the hollow space and, - as a result, of moving the movable component (107, 225, 333) into the extended position, wherein the underwater body (101, 201, 301) is designed such that - the fluid (121, 221, 321) in the hollow space hardens and - the hardened fluid (121, 221, 321) holds the movable component (107, 225, 333) in the hollow space in the extended position;. - the fluid (121, 221, 321) in the hollow space hardens and - the hardened fluid (121, 221, 321) holds the movable component (107, 225, 333) in the hollow space in the extended position;. characterized in that the hollow space is connected to the movable component (107, 225, 333) via a piston cylinder unit and the hollow space is formed in a chamber of the piston cylinder unit such that the expansion means conducts the liquid fluid into the chamber as to displace a piston of the piston-cylinder unit.