A sea of ancient water tainted by the Cold War is creeping deep under the volcanic peaks, dry lake beds and pinyon pine forests covering a vast tract of Nevada.Over 41 years, the federal government detonated 921 nuclear warheads underground at the Nevada Test Site, 75 miles northeast of Las Vegas. Each explosion deposited a toxic load of radioactivity into the ground and, in some cases, directly into aquifers.During the era of weapons testing, Nevada embraced its role almost like a patriotic duty. There seemed to be no better use for an empty desert. But today, as Nevada faces a water crisis and a population boom, state officials are taking a new measure of the damage."It is one of the largest resource losses in the country," said Thomas S. Buqo, a Nevada hydrogeologist. "Nobody thought to say, 'You are destroying a natural resource.' "