In the remote desert where China detonated its first atomic bomb nearly 60 years ago, a drilling rig recently bored a deep vertical shaft that is estimated to plunge down at least a third of a mile. It is the strongest evidence yet that Beijing is weighing whether to test a new generation of nuclear arms that could increase the lethality of its rapidly expanding missile force.

For years, U.S. government reports and independent experts have expressed vague concerns about the old base, Lop Nur. The reports point to possible preparations for year-round operations and a “lack of transparency.”

Now, however, waves of satellite images reveal that the military base has newly drilled boreholes — ideal for bottling up firestorms of deadly radiation from large nuclear blasts — as well as hundreds of other upgrades and expansions.

“All the evidence points to China making preparations that would let it resume nuclear tests,” said Tong Zhao, a nuclear expert at the Carnegie Endowment for International Peace.

Siegfried S. Hecker, a former director of the Los Alamos weapons laboratory in New Mexico, described Lop Nur’s rebuilding as unusual. “The Russians and Americans have continued activity at their test sites,” he said, “but nothing like this.”

Analysts say the activity at Lop Nur signals a wide modernization of China’s nuclear establishment, warning that it could speed arms buildups and spark a new age of atomic rivalry.

They add that China’s moves, along with those of other nuclear powers, could undermine the global test ban that began in 1996. The world’s atomic powers signed it after the Cold War as a way to curb a costly nuclear arms race that was spinning out of control.

The new evidence at Lop Nur was uncovered by Renny Babiarz, a former analyst at the National Geospatial-Intelligence Agency, an arm of the Pentagon. An expert on satellite reconnaissance as well as Beijing’s nuclear program, Babiarz says that detonations in the deep shafts could accelerate an effort to perfect new types of nuclear arms for the country’s fast-growing arsenal. Independent experts who have examined the satellite imagery and Babiarz’s analyses share his concerns.

The activity at Lop Nor comes at one of the most sensitive moments in U.S.-China relations. President Joe Biden has said he’s trying to “stabilize” an increasingly contentious relationship and, at a summit meeting last month with Xi Jinping, China’s leader, sought a measure of accord.

U.S. intelligence officials say they’ve followed Lop Nur’s revival for years. While the construction is obvious, they say, its purpose is not. China could be preparing for a nuclear test, they concede. But they add that Xi may not intend to move ahead unless the United States or Russia go first. The officials say Xi could be hedging his bets, drilling the deep vertical shafts so that, if necessary, China can act quickly.

Last week, the Foreign Ministry in Beijing responded to questions about upgrades at Lop Nur, dismissing them in a statement as “clutching at shadows, groundlessly whipping up a ‘China nuclear threat.’” It called such claims “utterly irresponsible.”

The ministry also emphasized Beijing’s commitment to observing the nuclear test ban — a position, it added, that “has won high praise from the international community.” China, it said, will spare “no effort to realize the noble aspiration of comprehensively banning and totally eradicating nuclear weapons.”

A Military Base’s Rebirth

Babiarz, the former intelligence analyst, wrote his doctoral thesis on the roots of China’s nuclear program. He now runs the operational arm of a company that analyzes civilian satellite imagery, teaches a course on geospatial analysis at Johns Hopkins University and recently co-wrote a textbook on the interpretation of satellite images.

Since leaving government service, Babiarz has spent much time analyzing images of Lop Nur. In assessing the base, he has looked at not only the most up-to-date imagery but earlier views that provide a visual history of the site’s development.

In a report this year, he documented a new test area covering roughly 10 square miles. The barren site possesses deep boreholes, Babiarz said, as well as new roads, power lines, an electrical substation and a support area with multiple buildings.

“The overall indications are that they’re preparing to test,” he said in an interview.

Lop Nur is a sprawling military base, roughly the land area of Virginia, in the arid Xinjiang region of China’s far west. Chinese accounts say the area was chosen for nuclear tests because it was so barren and isolated, without any permanent residents. But the broader Xinjiang region is home to the Uyghurs, a largely Muslim ethnic group that has recently endured mass detentions and pervasive security controls.

The Uyghurs have long protested health threats from nuclear tests at the site, which began in 1964 after Mao Zedong decided to build the bomb. The early tests were capped by mushroom clouds and radioactive fallout. China conducted its first underground nuclear test in 1969.

At first, China used shallow horizontal tunnels. It was a relative latecomer to drilling vertical shafts that were deep enough to reliably contain the deadly radiation, especially for large blasts. Its first such shaft test occurred in 1978.

Mao famously disparaged the bomb as a “paper tiger.” China thus built a nuclear arsenal smaller than the combined atomic forces of France and the United Kingdom. After the Cold War, the Lop Nur test site ended its large blasts and became a relative backwater.

That began to change after 2012, when Xi came to power. The Chinese leader saw the Rocket Force, which he created in late 2015, as one of his glories. The elite organization, the custodian of China’s nuclear weapons, embodied Xi’s ambitions to elevate his country as a respected, and feared, great power ready to stand up to the United States.

Xi’s political rise, it turns out, coincided with Lop Nur’s rebirth. The secretive activity was publicly disclosed by means of a new kind of open surveillance.

A Sleuth and His Discoveries

The most powerful civilian imaging satellites that orbit the Earth can distinguish objects on the ground as small as a foot in diameter. From hundreds of miles up, the spacecraft and their telescopes can discern people, types of vehicles and even aircraft tail numbers, analysts say.

Babiarz’s analyses of civilian imagery over several years tell the story of Lop Nur’s upgrades. By 2017, an old site with a handful of buildings had turned into a slick, ultramodern complex ringed by security fences. Its new structures included a bunker protected by earthen berms and lightning arresters, making it ideal for handling high explosives.

The site was suspicious but ambiguous. Nuclear arms use conventional explosives to start their chain reactions. But so do subcritical tests — lesser experiments that the global test ban treaty allows. Conservatively, Babiarz called the complex a possible site for the preparation of atomic devices.

By 2018, the next big project at Lop Nur was the rapid expansion of a giant air base whose main runway was 3 miles long. Satellite images showed a dozen large buildings under construction.

In parallel, the Trump administration declared in 2018 that “the United States must remain ready to resume nuclear testing.” In 2019, the Chinese Foreign Ministry criticized the U.S. as undermining not only the test ban but “global strategic security and stability.”

Even so, senior Trump officials in May 2020 discussed the merits of an atomic restart. News of the meeting leaked. Soon after, Republicans on the Senate Armed Services Committee approved spending $10 million to reduce the time it would take to conduct an atomic test.

Amid the heightened tensions, Babiarz discovered that China had begun to revamp and expand Lop Nur’s test zones — initially, its horizontal tunnels. Fairly close to the surface, they had shaken with modest test explosions during the Cold War. A 2020 satellite image showed a main tunnel undergoing extensive new digging and construction, its entrance surrounded by trucks, bulldozers and piles of excavated dirt.

Like the high-security site, however, the tunnels were ambiguous. Their rocky depths could be used to bottle up blasts from small atomic bombs or large subcritical tests.

Then, unexpectedly, the clues turned more troubling. Early in 2021, Babiarz was examining new Lop Nur imagery when he noticed large pieces of road-making equipment: a grader and a front-end loader.

“They were out in the middle of nowhere,” he recalled.

New Road Leads to Drilling Rig

Babiarz followed the new road. Over a dozen miles, it led from a flat zone used decades ago for vertical shaft detonations to a hilly region undergoing fast development.

In a labyrinth of canyons he found, to his surprise, a large drilling rig being set up. It was almost 90 feet tall. That was July 2021. The next month, he obtained a new image showing not only the derrick but a stack of drill pipes and an adjacent pit of lubricating fluid for keeping the drill bit moving ever deeper. From the clues, Babiarz estimated that the borehole was meant to go down at least one-third of a mile.

In comparison, the maximum depth of a vertical shaft at the U.S. government’s Nevada test site is 2,500 feet, or about a half mile. So the estimated depth of China’s borehole appears to fall within the American range.

Unlike Lop Nur’s flat areas, the new zone’s rugged terrain provided good concealment for large gear. Despite this, Babiarz found a second drill site last year. It “was tucked deeper into the hilly terrain,” he said, and the rig’s support gear had been meticulously covered.

To atomic experts, the deep holes seemed to have been designed to contain large nuclear tests. “We never did subcritical tests in vertical shafts,” said Patrick Rowe, a former director of drilling operations at the Nevada test site. “It didn’t make sense because drilling the holes was so expensive.”

The surprise discovery of the drill rigs prompted Babiarz to look wider. Recently, he zeroed in on a sprawling support base. By examining its historical imagery, he found that more than 30 of its buildings were major renovations or new construction.

Unexpectedly, he also found what appeared to be a training site for the shaft drillers. The biggest rig seemed similar to the tall one he had observed a hundred miles away in the new test zone.

“The bottom line is that it’s quite active,” Babiarz said of the military base.

Bluster and Brinkmanship

Nuclear experts say they see no signs of an imminent Chinese test and argue that Beijing may do nothing. The rebuilding of the military base could simply be a warning to the West, they say. Chinese experts have suggested as much.

Whether China conducts a nuclear test might also depend on what its rivals do. Signals sent recently by Russia and earlier by American administrations may worry Beijing.

Richard L. Garwin, a prominent nuclear physicist who has often visited China and met with its top scientists, said Lop Nur’s rebuilding represents a technological hedge. “They don’t want to be caught flat-footed in case somebody else goes first,” he said.

Zhao of the Carnegie Endowment agreed: “China feels it needs to prepare for the worst-case scenario.”

Stephen M. Younger, a former director of Sandia National Laboratories, said it was not inconceivable that China would unilaterally conduct a test to make a statement.

Some analysts argue that China may not be planning large blasts but instead ramping up its program of subcritical experiments, which stop short of producing self-sustaining chain reactions. Both Russia and the United States use them to assess their existing arms.

Other analysts disagree, arguing that China’s fleets of new bombers, submarines and missile silos herald a push for new armaments.

China could field 1,500 nuclear warheads by 2035 at its current pace of force expansion, the Pentagon has projected. That figure would be a fivefold increase from the “minimum deterrent” that China possessed for more than half a century.

American experts say Chinese scientists are now planning the particular arms they see as best suited for that buildup and may learn much from test explosions — unlike Moscow and Washington, which conducted far more blasts during the Cold War.

“I know they believe they’re behind,” said Terry C. Wallace, a former Los Alamos director who has long studied China’s program of nuclear experimentation. “They may feel that they’re so far behind in testing knowledge that they want to do something exotic.”

A Question of Risk and Reward

Nuclear testing allows scientists to uncover flaws and fine-tune new weapon designs. During the Cold War, China conducted 45 test explosions. In comparison, France set off 210, Russia 715 and the United States 1,030.

The test ban treaty, although signed by 187 nations, never entered into force because China, the United States and six other nations failed to ratify it. Even so, analysts have long argued that it favors Washington because it bars pact-abiding rivals from making their nuclear arms more advanced.

What China wants most, experts say, is miniaturization. With new, more accurate missiles that pinpoint targets, its scientists can reduce the power, size and cost of warheads.

“They have perfectly serviceable nuclear weapons,” said Garwin, who is credited with designing the world’s first hydrogen bomb. “But they might want to make them smaller.”

Experts say miniaturization could make China’s submarine missiles more deadly. They’re judged to be carrying up to three warheads each. In contrast, the main American submarine missile, the Trident II, carries up to eight.

Miniaturization could also aid China’s development of hypersonic warheads that would zig and zag to evade U.S. defenses. According to the Pentagon, Beijing wants to give the weapons a nuclear edge.

This fall, global civil society leaders warned of “growing threats” to the test ban and suggested ways to reinforce the taboo against experimental blasts.

While Chinese officials are quick to deny any testing plans, they’re even faster to criticize Washington.

“If you truly care” about the test ban, the Foreign Ministry said in its statement, “you should pay serious attention to the position of the United States.” For instance, Chinese experts accuse Washington of a double standard in backing the global ban while seeking technologies that can sidestep its prohibitions.

Last year, American scientists announced a fusion breakthrough that was hailed as a step toward abundant clean energy. The next day, Global Times, China’s nationalistic Communist Party tabloid, highlighted a little-known fact about the giant laser: Its main job is to advance nuclear arms development.

Of late, China has pointed to the W93 — the first new American nuclear warhead since the Cold War, now in development — as an example of U.S. nuclear strides. The military says the innovative weapon will require no test detonations.

Even so, Chinese experts cast the threat of test resumption as coming from the United States. In September, the military affairs channel of China’s state television network ran a program in which Teng Jianqun, a former military officer, said Washington was “trying to break through the restrictions.”

American experts, in contrast, cast Lop Nur’s modernization as a sign of just how far the Chinese may be willing to go.

“We have to realize that they had a conservative posture,” said Wallace, the former Los Alamos director. “That’s changing.”

This article originally appeared in The New York Times.