The water which bubbles up from the ground at Bath, fell as rain on the nearby Mendip Hills. It percolates down through limestone aquifers to a depth of between and where geothermal energy raises the water temperature to between and . Under pressure, the heated water rises to the surface along fissures and faults in the limestone. This process is similar to an artificial one known as Enhanced Geothermal System which also makes use of the high pressures and temperatures below the Earth's crust. Hot water at a temperature of rises here at the rate of every day, from a geological fault (the Pennyquick fault).