Abstract Extremely high permeability has been found at 410 m depth in the Weardale Granite, UK, during exploratory drilling for low-enthalpy geothermal resources. The Eastgate Borehole was sunk down the axis of an ancient, sub-vertical hydrothermal vein structure (the Slitt Vein), proving a transmissivity in excess of 4000 darcy m (= 3 × 10⁻⁹ m² m), which is one of the highest values ever measured deep within a granite intrusion. Sustained hydraulic testing over 24 h showed that this transmissivity was not local to the borehole: sustained flows and steadying of drawdown suggest that this transmissivity persists along the strike of the Slitt Vein structure. Although this finding is encouraging in relation to the prospects for geothermal energy development in this and similar granites, it raises disquieting issues in relation to assumptions commonly made during planning for radioactive waste isolation concerning the feasible maximum permeability likely to be encountered in granite.