Method to construct a sea-floor-based vertical, cable-braced, reinforced-concrete (or all-steel) megastructure, for large cities infrastructure, which is made (or assembled) by mobile, robotized factories. Such a system is comprised of a new method for realizing an underwater continuous footing system which produces a circular "dam" able to resist external, dynamic hydraulic seawater pressure in order to recover new land from unused shallow sea-floors. (This technique is thought to be applicable to prison and penitentiary construction.)