In 1990, a group of Soviet scientists moved to Leonard, Oklahoma. They came as watchdogs, to ensure that the United States obeyed the nuclear Threshold Test Ban Treaty and did not secretly explode atomic bombs below ground. The Soviets needed a profoundly quiet place to observe vibrations in the region, because tremors might confuse their measurements. And in the United States, there were few places as seismically tranquil as Oklahoma. Not any more, thanks to fracking.