In his Opinion article, Domenico Giardini (Nature 462, 848–849; 2009) calls for a better understanding of earthquake risk in pursuing deep geothermal energy using an enhanced geothermal system (EGS). However, earthquakes are only part of the problem in trying to tap Earth's internal heat as an alternative clean-energy source. Poorly understood geology is a bigger obstacle.

Geological anomalies halted an EGS demonstration project in Geysers, north California, sponsored by the US Department of Energy. After months of drilling last year, the California-based company AltaRock Energy was unable to penetrate the formation capping the hot rocks that it was targeting 4 km below the surface. Similar frustrations were encountered during EGS drilling projects at Paralana and the Cooper Basin, both in South Australia.

Depths of 3–10 km are optimal for geothermal exploitation because they are extremely hot and accessible to modern drilling techniques. But even the most carefully planned scientific drilling operations can be victims of geological surprise. For example, scientists at the German Continental Deep Drilling Programme site east of Nuremburg predicted a change in rock type at a depth of about 7 km, corresponding to the boundary between two tectonic plates that collided 320 million years ago. But no such boundary was evident, even after drilling to more than 9 km (B. Yardley Nature 389, 792–793; 1997).

The United States and China are together injecting US$150 million over the next 5 years into a joint Clean Energy Research Center. Meanwhile, China is launching its ambitious Deep Exploration Technology and Experimentation project, SinoProbe, to locate mineral resources and to find out more about earthquakes and volcanism (S. Dong and T. Li Acta Geologica Sinica 83, 895–909; 2009). Together, these ventures should improve our geological understanding and enable us to find drilling sites that are less prone to triggering earthquakes.