The exploration well WD-1a was drilled to a depth of 3729 m in the Kakkonda geothermal field, northeast Japan, using efficient borehole cooling techniques. The well penetrated an entire shallow hydrothermal convection zone, an entire contact metamorphic aureole and part of a neo-granitic pluton. The recovered temperature of the well indicates a boiling point-controlled profile up to 380 degrees C to a depth of 3100 m, and a conduction-controlled profile with a very high gradient from 3100 m to the bottom of the hale, where the temperature is 500 degrees C. WD-1a may be the first geothermal well that encountered 500 degrees C, which exceeds the conventional hydrostatic boiling-point curve. An inflection point of the temperature-depth profile at 3100 m and about 380 degrees C reflects the brittle-plastic boundary. The brittle-plastic boundary constrains the maximum depth of fracture formation, and the fracture distribution constrains the maximum depth of hydrothermal convection.