In the video, the researchers scale the cavern wall, their headlamps ghostly blue.
"If our skin is exposed, it can easily come in contact with bat excrement and contaminated matter, which means this is quite risky," says Tian Junhua, one of the bat hunters.
"We have to live for several days in the cave . . ." he continues, as the soundtrack amps up the drama. "There's no cellphone signal, no supplies. It's truly scary."
The video was released by national science authorities and Chinese state broadcaster CCTV on Dec. 10, 2019, and circulated on social media. It's a high-quality production, designed to promote China's world-leading viral research. Aired around the time Wuhan residents began turning up at hospitals with mysterious respiratory ailments, it also offers a rare glimpse of field conditions on the eve of the pandemic.
Tian and his team from the Wuhan Center for Disease Control and Prevention are filmed catching horseshoe and pipistrelle bats and collecting samples of guano, in search of new bat-borne diseases and the basis of new vaccines. Tian talks about the need for caution. "It is while discovering new viruses that we are most at risk of infection," he says, though he is shown handling sample vials without wearing full protective gear.
The video is perhaps even more notable for what it doesn't reveal. Nothing is known outside China about the science gleaned from that expedition by the Wuhan CDC - the same agency that oversaw China's early pandemic response. The team has not disclosed what viruses, if any, it found in the cave, or even when the mission took place. According to a World Health Organization report released in March, the Wuhan CDC denied any storage or laboratory activities involving bat viruses before the coronavirus outbreak - a stance hard to reconcile with Tian's boasts in the video about having visited dozens of bat caves and studied 300 types of virus vectors.
Tian has not spoken publicly for more than a year.
Video: Wuhan lab staff reportedly sought care before pandemic
The silencing of scientists, the blanket denials, the careful guarding of raw data and biological samples - these elements have been emblematic of the approach by Chinese authorities at every stage of the coronavirus outbreak. And they continue to obstruct the world's ability to get answers.
There is no direct evidence linking Tian's team, or a rival group of bat-disease specialists at the Wuhan Institute of Virology, to the coronavirus outbreak. Nor is there more than circumstantial evidence to support any theory of the pandemic's origin. Many scientists say the most likely path is that the virus spread in nature and jumped from animals to humans. But that belief is largely based on how other coronaviruses have originated, not what is known about this case.
The lack of clarity is not in itself alarming at this point in an investigation of virus origins - in the case of severe acute respiratory syndrome (SARS), scientists were fairly quick to identify civet cats as the intermediate host, though it took years to find the bat population that harbored the building blocks of the virus. But the WHO chief, the Biden administration, other governments and scientists around the world have rebuked China for not making this investigation any easier.
Last week, President Joe Biden directed U.S. intelligence agencies to redouble efforts to determine the pandemic's origin, including whether it could have emerged "from a laboratory accident."
China's Foreign Ministry protested, with a spokesman saying Monday that Beijing supported scientific inquiry on the question but not "an intelligence-led investigation stoking confrontation."
Chinese authorities weren't much more receptive of the international team commissioned by the WHO. Negotiations over the arrangements delayed the team from getting to Wuhan until more than a year after doctors first raised concerns there. Once on the ground, the international experts were given limited access. They visited the market linked to early coronavirus cases - but it had been shut for a year and its contents long ago removed. Their visit to the Wuhan Institute of Virology lasted three hours. In general, they had to satisfy themselves with data that was in large part collected by Chinese scientists before the trip.
The result was a report that didn't significantly advance the world's understanding of how the pandemic came about.
The report lent credence to China's preferred theory that the virus could have come from overseas, possibly via frozen food imports - though Beijing has presented little support for that. On the question of a possible lab leak, the report concluded that pathway was "extremely unlikely."
Analysts have accused China of inappropriately influencing the team's conclusions.
For Beijing, there are huge political risks if it loses control of the narrative, said Yanzhong Huang, a senior fellow for global health at the Council on Foreign Relations.
"They want to portray China as successful at containing the spread, at being so cooperative and sharing information," he said. "This lab-escape theory, which indicates that China was the cause of the problem, makes the official narrative unreliable."
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The lack of transparency has fanned both speculative and legitimate questions. Within the realm of U.S. politics, entertaining the lab-leak theory is less toxic now for those on the left than when it was embraced by the Trump administration as another way to bash China. And in the world of science, some of the early arguments against a lab leak have been countered by respected researchers, launching a genuine debate.
"We can't even begin to talk about a consensus other than a consensus that we don't know," said David Relman, a Stanford University microbiologist. "We have nothing like the amount of data we need."
Relman is one of 18 scientists who wrote an open letter last month urging serious consideration of the possibility that the pandemic originated with a lab accident.
"Any lab that handles a significant number of samples from bats or other potential coronavirus host species, or performs engineering on coronaviruses," should be considered, he said in an interview this week with The Washington Post.
Wuhan's two rival teams of exotic bat disease specialists are now under renewed scrutiny. Tian's team at the Wuhan CDC and Shi Zhengli's at the Wuhan Institute of Virology (WIV) have both drawn criticism for a willingness to compromise safety, as they raced to make discoveries.
The Wuhan CDC and WIV did not reply to requests for comment, nor did Tian or Shi. An unnamed staffer who answered the phone Tuesday at the Wuhan CDC said the center did not accept interviews and directed questions to the National Health Commission. The NHC did not reply to a request for comment.
China's Foreign Ministry and the Chinese Embassy in Washington declined to answer questions for this article.
Biden has ordered U.S. intelligence agencies to report back in 90 days.
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Dubbed China's "Bat Woman," Shi has been the country's star coronavirus researcher, identifying multiple new SARS-like viruses over the years and giving prescient warnings about the risk of a new pandemic. Her lab at the Wuhan Institute of Virology also discovered and studied the closest-known cousin to the coronavirus, though scientists note it would require decades of natural evolution to bridge the two.
The WHO report says Shi told the visiting team that all field work is done with full protective equipment - which experts say would include N95 masks, Tyvek bodysuits, goggles and gloves. But in a June 2018 lecture, Shi said workers don't do that in practice.
"In most cases, we'd wear simpler protection, and it's okay," she said on a Chinese TED Talks-like program called Yixi. She said that's because most bat-borne diseases cannot infect humans directly, only through an intermediary animal.
To illustrate, she showed slides of her team spreading out nets in a cave to catch bats and sorting samples afterward. Some wear thin surgical masks and rubber gloves as they work, while others are unmasked with bare hands.
"Under what situation would we increase our protection? For instance, when there are too many bats in the cave, and lots of dust even as you're entering," she said.
Shi kept a low profile at the pandemic's onset, with her occasional interventions reflecting the pressures on her team. On Feb. 2, 2020, she posted to friends on WeChat that she "swore on her life" her lab was not involved in the outbreak. Three months later, she followed up with a post denying rumors she had defected to the West with intelligence files, as reported in the state-run Global Times.
As questions about the WIV intensified, Shi wrote a lengthy statement to Science magazine in July. She wrote that it was impossible for the coronavirus to have come from the WIV, as her team had not come across this strain in its research, and all staffers had tested negative for coronavirus antibodies. Shi said her team had "never been in contact with or studied this virus, nor [knew] of its existence" before the pandemic.
But she also acknowledged the lab had not done genome sequencing of all its samples, because of financial and personnel constraints. She declined to say how many samples remained unsequenced.
Suspicions about the lab were elevated in January, after the U.S. State Department said it had "reason to believe" that several WIV researchers showed symptoms similar to those of covid-19 in the fall of 2019. The Wall Street Journal reported last month, citing an intelligence report, that three WIV researchers were sick enough to go to a hospital.
WIV's lab director, Yuan Zhiming, called it a "complete lie" in state media.
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The Wuhan CDC lab has received less attention.
The WHO team's account of its visit to the facility doesn't include notes on research safety. The report mentions that the lab moved on Dec. 2, 2019, to a location close to the Huanan market linked to early coronavirus cases. "Such moves can be disruptive for the operations of any laboratory," the team writes, adding that no such disruptions had been reported by the lab.
Tian is by title an associate chief technician in the Wuhan CDC's pest-control department, but he has a reputation as a swaggering adventurer in his work with bats and insects.
"He will often go to places other people can't find to look for the samples he needs," a colleague, Liu Jing, said on local TV in January 2020. "He can say quite confidently that he can capture things other people can't."
Using nets and traps, Tian's team caught 155 bats in Hubei, his home province, and hundreds more in other regions for a 2013 study. He was part of a team that discovered 1,445 new RNA viruses in invertebrate animals, published in the elite journal Nature in 2016.
The Wuhan CDC organized an internal meeting titled "Learning from the Achievements of Tian Junhua."
"No one remembers how many mountains he climbed, how many rivers he forged, how many bat caves he explored, how many cowpens and pigsties he crouched in, how many trash heaps he dug through," the CDC's account of the event said. "He enjoys it."
He has also admitted the occasional safety mishap.
In 2017, Tian told the state-run Wuhan Evening News he once forgot personal protective equipment and was splattered with bat urine, leading him to quarantine at home for two weeks. On multiple occasions, bat blood squirted onto his skin while he was trying to grasp the animals with a clamp, he told the paper.
On Feb. 3, 2020, Tian's team landed in the pages of Nature again, with an early clinical account of a coronavirus patient in Wuhan. Their paper pointed to bats as a possible host.
But as the coronavirus spread, Tian went quiet. The state-run newspaper Health Times cited an anonymous source in March 2020 saying Tian was not infected with coronavirus, and that he was in poor spirits because of speculation over whether he was patient zero.
The Health Times said it reached Tian by phone and he declined to answer questions - an unusual note of tension in a state media profile. Tian has not spoken publicly since.
It highlights a challenge facing independent probes: Many of those in Wuhan who may hold key information are quiet, whether under official pressure or by personal choice.
Tian has continued his research while staying out of the public eye. A paper he co-authored in August discussed genetic diversity in ticks. One in November focused on antibiotic-resistant genes in fish.
He has not said or published anything more about bats.
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The Washington Post's Pei Lin Wu and Alicia Chen contributed to this report.