A controversial method of drilling for natural gas, called fracking, has boomed in recent years—as have concerns over its potential to cause environmental contamination and harm human health. But a major review of the practice uncovered no signs that it is causing trouble below ground. “We found no direct evidence that fracking itself has contaminated groundwater,” said Charles Groat of the University of Texas, Austin, who led the study.

The report, released at the annual meeting of the American Association for the Advancement of Science (which publishes ScienceNOW), doesn’t give this form of natural gas extraction a clean bill of health.