Industrial exploitation is causing clearly-measurable changes in Earth structure at The Geysers geothermal area, California. Production at The Geysers peaked in the late 1980s at ~3.5¿103 kg s-1 of steam and 1800 MW of electricity. It subsequently decreased by about 10% per year [Barker et al., 1992> because of declining reservoir pressure. The steam reservoir coincides with a strong negative anomaly (~0.16,~9%) in the compressional-to-shear seismic wave speed ratio &ngr;P/&ngr;S, consistent with the expected effects of low-pressure vapor-phase pore fluid [Julian et al., 1996>. Between 1991 and 1994 this anomaly increased in amplitude by up to about 0.07 (~4%). This is consistent with the expected effects of continued pressure reduction and conversion of pore water to steam as a result of exploitation. These unique results show that &ngr;P/&ngr;S tomography can easily detect saturation changes caused by exploitation of reservoirs, and is a potentially valuable technique for monitoring environmental change. They also provide geophysical observational evidence that geothermal energy is not a renewable energy source.¿ 1997 American Geophysical Union