Abstract: The present invention refers to a underwater unit (1) for a hydroelectric power plant and a modular hydroelectric power plant (2) comprising a plurality of such units. The unit (1) according to the invention uses the principle of functioning of the traditional hydroelectric power plants and comprises a penstock (200) which involves a “head” of at least 100 m, and preferably of about 150-300 m, and carries elevated kinetic energy water masses from the surface of a water basin down to one or more turbines (301a-301f) provided in depth under the surface of the basin itself, so as to transfer the kinetic energy from the water to said turbines and to transform then the kinetic energy into electric power. Thanks to the use of variable volume discharge tanks (313a,313b) positioned downstream the turbines, it is possible to easily and effectively return the water masses used for the actuation of turbines themselves to the surrounding environment.