Extracting heat from very hot, dryrocks presents a more difficult problem: the rocks must be fractured to permit the circulation of water, and the water must be provided artificially.
Geothermal power stations tap aquifers heated by contact with hotrocks in volcanic regions-or, in hot but dry spots, they pump water past such rocks to heat it up.
Fracking is the technique behind the new "enhanced geothermal systems" that extract energy from rocks which are hot enough, but too dry, to produce steam.