Abstract: The invention relates to a thermal energy storage with at least one thermal energy storage volume. The thermal energy storage comprises at least one primary borehole extending from ground level to a first predetermined depth in a rock body; at least one set of secondary boreholes located around the at least one primary borehole; and at least an upper and a lower fracture plane extending in a radial and/or oblique plane from the at least one primary borehole towards adjacent secondary boreholes. At least one fracture plane permits a hydraulic flow between at least one of the secondary boreholes and the primary borehole. Each thermal energy storage volume is defined by one set of secondary boreholes and its upper and lower fracture planes. The set of secondary boreholes diverge away from the at least one primary borehole at each fractured plane level, without intersecting the at least one primary borehole.