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YUCCA MOUNTAIN IS A BARREN, flat-topped ridge rising out of the Nevada desert. Located 100 miles northwest of Las Vegas, it stands amid a desolate expanse that is broken every so often by a Joshua tree or yucca plant. The mountain rises some 1,500 feet and runs six miles or so. Within two decades it could hold the most dangerous nuclear facility in the world.

The Government plans to build a dump inside Yucca Mountain for the most highly radioactive nuclear waste in the nation. The job is considered urgent. More than four decades after the start of the nuclear era, there is no permanent place to put the vast and rapidly growing accumulation of the most lethal of America’s nuclear refuse — the spent fuel rods and radioactive debris that constitute a largely forgotten legacy of this country’s conquest of the atom. The nation has sites for the storage of low-level waste — and one, under construction in Carlsbad, N.M., for some higher-level military byproducts — but none for the most deadly substances of all.

The Government wants a final burial place for these items, now stored at Federal sites for nuclear weapons production and at many of the nation’s 111 operating nuclear power plants. It wants a repository that will keep them isolated from the earth’s environment for 10,000 years, until radioactive decay has rendered them less hazardous than they are today.

Yucca Mountain is the last candidate, selected after a stormy, decades-long, multibillion-dollar search that crisscrossed much of the nation and eliminated dozens of other potential sites, often for reasons of politics rather than science. Today, the site’s 1,800 federal and contract workers feel great pressure to stay on schedule in assessing the site’s suitability and planning the repository, which would be a labyrinth of tunnels up to 115 miles long. Its cost is expected to be up to $15 billion, some $1 billion of which has already been spent. If everything goes according to plan, it will open for business in 2010.

At first glance, Yucca Mountain seems perfect for the job. Its main virtue is aridity. The Nevada desert is one of the driest places in the United States. The water table beneath Yucca Mountain is unusually low, a third of a mile down. By contrast, the water table in the East can be within a dozen feet of the surface. In theory, canisters of radioactive waste (which are very hot both physically and radioactively) could be deposited deep inside Yucca Mountain and still be hundreds of feet above the water table, safe and dry for thousands of years, unlikely ever to contact or contaminate ground water, unlikely ever to spread radioactivity.

One scientist, however, has quietly but persistently warned that this vision of a safe repository is little more than a delusion.

Jerry S. Szymanski (pronounced sha-MAN-ski) is a geologist who works on the Yucca Mountain project for the United States Department of Energy, which is in charge of evaluating the site and would run the repository. For years, he has argued that ground water under the mountain could eventually well up, flood the facility and prompt a calamity of vast proportions. The geological action is easy to visualize. Crustal stresses in the area slowly open fractures and faults under and within the mountain. Water seeps into them. An earthquake occurs, compressing the fractures and forcing the ground water upward into the dump. As the inrushing water comes into contact with the hot canisters of nuclear waste, the water is vaporized, threatening to cause explosions, ruptures and the release of radioactivity.