The sheep that Su Duoguan tends in the rocky pastures outside this quiet town on the Tibetan plateau have a different sell-by date than most. He knows from long experience that he has one year to raise them before their teeth turn black and break, leaving his flock unable even to chew grass.
"All along this river, the sheep have the same problem," the 49-year-old shepherd says, pointing at a narrow creek feeding into a man-made reservoir on the edge of Xihai. "It's been like this for a long time."
Other shepherds in the area say they have the same problem. "I took my sheep to a veterinarian. He told me the teeth were turning black because of radiation," says 30-year-old Zhang Binting, squatting at the side of the road, keeping a passive eye on his flock of 100.
The sources of that radiation are likely all around Xihai, which sits 3,000 metres above sea level on the Tibetan plateau. Proudly labelled China's "Atomic City," it was here that China's first atomic and hydrogen bombs were designed and developed amid extreme secrecy in the 1950s and 1960s. The project, dear to Chairman Mao Zedong's heart, was called the Ninth Academy. The research here led to the detonation of China's first atomic bomb in 1964.
Mr. Su's and Mr. Zhang's sheep today graze on land down a short hill from what was then known as Factory No. 1 - the region's chief nuclear-research facility - and which today is a functioning power plant.
The shepherds say they sell their sick sheep to butchers once they lose their teeth, but have no idea where the meat ends up.
That blasé attitude toward lingering radiation risks is common here. As the shepherds keep a passive eye on their flocks, half-naked children play in the same stream the shepherds say is contaminated. The surrounding Jinyintan grasslands, where the nuclear waste from that era is believed to be buried, is now a popular tourist spot, attracting those who want to experience a night under the stars on the Tibetan plateau. Traditional Tibetan tents sit a few metres away from the abandoned factories and watchtowers that played a key role in helping China enter the exclusive club of nuclear-armed nations.
Though tales are told here of workers incinerated in accidents during the 1960s and that the nuclear waste from that era is buried in a 20-square-metre pit not far from Xihai, the official line remains that no one ever died as a result of the nuclear experiments conducted here and the government says it has seen no signs of health or environmental problems stemming from the testing that took place here in the 1960s and 70s.
While Tibetan exile groups say that dozens of local herders - and perhaps many more - have died as a result of illnesses linked to radiation, most residents of Xihai seem happy to accept the government's assurance that there are no dangers associated with living in this Chinese Los Alamos.
But the testing regime seems less than strict. "Your question reminds me that maybe we should have radiation tests from now on," says Duan Shuiqiang, senior engineer at Qinghai province's Hydrology and Water Resource Survey Bureau, when asked if there was any radiation in the groundwater. Such tests weren't currently taking place, he says, but he was confident, based on anecdotal evidence, that the radiation from the Ninth Academy wasn't affecting anyone's health.
It's only in the past two decades that China has admitted that anything at all out of the ordinary took place at Xihai. The region was a closed military area until 1993, when it was opened up and its population began to swell to its current 12,000.
The Ninth Academy began its work in the late 1950s, after Chairman Mao decided that China needed a nuclear weapon to be taken seriously by its chief rivals, the United States and the Soviet Union. The detonation of China's first atomic weapon on Oct. 16, 1964, in the deserts of the country's westernmost province of Xinjiang caught U.S. intelligence analysts by surprise. Unaware of the progress being made in secret at the Ninth Academy, they had believed China was years away from developing its own weapon.
In the centre of Xihai stands a nondescript building that for decades was believed to be an ordinary post office. But a secret door leads to a staircase leading down to a warren of rooms nine metres beneath the floor of the post office - which was presumably considered a safe enough depth in the event of an accident on the surface - that was once the headquarters of the Ninth Academy.
For 20 yuan (about $3), tourists can now climb down and see the Spartan rooms from where political officers directed the top-secret scientific effort taking place in the grasslands above them.
The main room contains only two desks, two chairs and two telephones connected directly to the scientists in the field and their bosses in Beijing. Gazing down on whoever manned the phones were oversized portraits of Mao and his deputy, premier Zhou Enlai.
Some of the equipment in the rooms has Russian lettering on it, donated by the Soviet Union before it later decided to cease helping its Communist neighbour achieve its nuclear ambition. A framed list of regulations gives a hint as to how high the stakes were for those who worked at the Ninth Academy.
"What you are not supposed to know, never ask. What you are not supposed to hear, never listen to," the sign warns. "What you do hear, never tell to others. You cannot speak in public about what you see and hear here."
Scientists who worked in the underground warren and in the seven workshops scattered around the surrounding grasslands were forbidden from telling even their families what they did. Those who broke the rules were reportedly shot for leaking information.
"I'm very proud of what happened here," said Song Yuling, a 22-year-old tour guide who moved to Xihai two years ago. "In that time, the leader of the Soviet Union [Nikita Khrushchev]said we Chinese shared our pants with apes. But after our atomic success, he stopped such talk."
But not all here are yet willing to talk about the Ninth Academy, at least not in front of foreigners. Earlier this year, the provincial government opened the sprawling "National Patriotic Education Base" in Xihai, a bunker-styled building that celebrates the moment and the place that China became a nuclear power. Each day, some 3,000 people go through the museum to gawk at exhibits such as a mock nuclear missile, white models of the original A-bomb and H-bomb that detonated here, and murals to the scientists who took part in the Ninth Academy.
But so far, displays are for Chinese eyes only. "No foreigners," a pair of unsmiling guards make clear as they block the entrance. "This is still a closed military zone."