Mary and Billy Boy Holiday bought their one-room house from a medicine man in 1967. They gave him $50, a sheep and a canvas tent.

For the most part, they were happy with the purchase. Their Navajo hogan was situated well, between a desert mesa and the trading-post road. The eight-sided dwelling proved stout and snug, with walls of stone and wood, and a green-shingle roof.

The single drawback was the bare dirt underfoot. So three years after moving in, the Holidays jumped at the chance to get a real floor. A federally funded program would pay for installation if they bought the materials. The Holidays couldn't afford to, but the contractor, a friend of theirs, had an idea.

He would use sand and crushed rock that had washed down from an old uranium mine in the mesa, one of hundreds throughout the Navajo reservation that once supplied the nation's nuclear weapons program. The waste material wouldn't cost a cent. "He said it made good concrete," Mary Holiday recalled.

As promised, the 6-inch slab was so smooth that the Holidays could lay their mattresses directly on it and enjoy a good night's sleep.

They didn't know their fine new floor was radioactive.

Fifty years ago, cancer rates on the reservation were so low that a medical journal published an article titled "Cancer immunity in the Navajo."

Back then, the contamination of the tribal homeland was just beginning. Mining companies were digging into one of the world's richest uranium deposits, in a reservation spanning parts of Arizona, New Mexico and Utah.

From 1944 to 1986, 3.9 million tons of uranium ore were chiseled and blasted from the mountains and plains. The mines provided uranium for the Manhattan Project, the top-secret effort to develop an atomic bomb, and for the weapons stockpile built up during the arms race with the Soviet Union.

Private companies operated the mines, but the U.S. government was the sole customer. The boom lasted through the early '60s. As the Cold War threat gradually diminished over the next two decades, more than 1,000 mines and four processing mills on tribal land shut down.

The companies often left behind radioactive waste piles and open tunnels and pits. Few bothered to fence the properties or post warning signs. Federal inspectors seldom intervened.

Over the decades, Navajos inhaled radioactive dust from the waste piles, borne aloft by fierce desert winds.

They drank contaminated water from abandoned pit mines that filled with rain. They watered their herds there, then butchered the animals and ate the meat.

Their children dug caves in piles of mill tailings and played in the spent mines.

And like the Holidays, many lived in homes silently pulsing with radiation.

Today, there is no talk of cancer immunity in the Navajos.

The cancer death rate on the reservation — historically much lower than that of the general U.S. population — doubled from the early 1970s to the late 1990s, according to Indian Health Service data. The overall U.S. cancer death rate declined slightly over the same period.

Though no definitive link has been established, researchers say exposure to mining byproducts in the soil, air and water almost certainly contributed to the increase in Navajo cancer mortality.

The government has never conducted a comprehensive study of the health effects of uranium mining on the reservation. But individual scientists working on their own have documented sharply elevated cancer rates near old mines and mills. High concentrations of uranium, arsenic and other heavy metals have been found in one out of five drinking-water sources sampled.