Abstract: A borehole reservoir includes a plurality of vertical boreholes (5, 6) in each of which there is arranged pipes (9, 17) for feeding water to and returning water from the boreholes.In order to avoid sealing the boreholes (5, 6) so as to prevent leakage when the water-circulating circuit is placed under pressure, all connections between the boreholes have, in accordance with the invention, been placed at a level beneath the ground-water level (2). This is achieved by providing spoke-like, horizontal boreholes (3, 4) from the central shaft (1), these boreholes intersecting the vertical boreholes (5, 6) located along the spokes and serving as supply and return lines for the water to and from the vertical boreholes. Since the pressure of the water within the boreholes is the same as that of the water externally of the boreholes, there will be no leakage from the boreholes (5, 6).