When Sixth Tone arrives at FAST after a two-hour drive from the provincial capital Guizhou, the crew is midway through one of the most challenging maintenance projects it has undertaken so far: replacing the enormous steel cables that hold FAST’s antenna in position.

It’s a complex task. The six cables are suspended from huge support towers that loom over the dish, around 30 stories in the air. Each is over half a kilometer long and weighs several tons.

“It’s our first time doing this, so we want to be extra careful and make sure there’s no human error in even the easiest steps,” Yao Rui, the engineer leading the maintenance project, tells Sixth Tone. “It’s not a difficult mission in terms of technique, but it has to be done perfectly.”

For Yao, it has been a tense few weeks. One loose bolt or tiny misstep could cause incalculable damage to the telescope, which cost about 1.2 billion yuan (then $180 million) to build.

The work, meanwhile, has to be completed amid a communications blackout. Every mobile base station within a 5-kilometer radius has been removed, to create a signal vacuum for the telescope.

To attract each other’s attention, the workers simply holler a name at the top of their lungs. If the colleague isn’t listening, other team members join the chorus.

The historic rainfall that has battered China through the summer hasn’t helped, either. Replacing the first cable alone took 13 days, Yao says, as the slippery surfaces and lightning storms made it too risky to continue working.

“Whenever it rains, we have to pause the work,” she says. “The observation team nudges us every day and asks when we can finish.”

The maintenance team is under intense pressure to get the job done within 60 days, according to Yao. For the Chinese authorities, FAST is simply too important to tolerate any delays.

The idea of building a radio telescope of this scale first took shape in the mid-’90s, when an international group of radio wave experts convened in Japan.

At the time, concerns were growing about the explosion in digital communication systems like cellphones. Scientists worried these would soon fill the Earth with artificial radio signals, making it impossible to identify those arriving from space. A superlarge telescope might be the only way to prevent humanity from being deafened by its own chatter.

China, spying an opportunity to become a world leader, decided to step up and build a facility in 2007. A few years later, Chinese scientists settled on the mountains where Sixth Tone is standing as the ideal location.

The jagged peaks are miles away from the nearest city, sheltering the facility from signal interference. The deep crater, meanwhile, fits FAST’s massive spherical dish almost perfectly.

Construction was finally completed in 2016. The telescope began trial operations soon after amid much media fanfare, with President Xi Jinping hailing FAST China’s new “eye in the sky.”

Since then, the facility has been quietly pursuing seven goals assigned by the Chinese Academy of Sciences, each of them designed to deepen knowledge of the cosmos and promote China’s space exploration program.

One of them is to detect pulsars — fast-spinning dying stars that emit pulses of radiation at such regular intervals, they’re sometimes called the “lighthouses of the universe.”

Astronomers expect pulsars can serve as vital reference points in studies of time and as navigation tools for spaceships. But their faint pulses are extremely difficult to detect. Before FAST, no domestic scientist had ever discovered a previously unknown pulsar.

The telescope is also searching for neutral hydrogen, miniscule particles that could help astrophysicists understand how the universe was formed. Hunting for signs of extraterrestrial intelligence is also on the list.

For the first four years, the team in Guizhou spent much of their time optimizing and debugging the facility. But the telescope has already made its presence felt.

As of January, it had discovered over 100 new pulsars. Chinese astronomers have predicted FAST will eventually double the number of pulsars known to humankind, which currently stands at around 2,000.

The engineers, however, rarely pay much attention to this research. They tend to focus on more prosaic matters.

“I’m not really interested in astronomy,” says Lei Zheng, a permanent member of the FAST maintenance team. “I focus more on what’s happening at the site and leave the work of making breakthroughs to the scientists.”

Astronomers are rare visitors here. A few times a year, one will drive up to the facility, clutching a bagful of hard drives. FAST can collect up to 38 gigabytes of data per second, so transmitting it over the internet is almost impossible.

Once the disks are filled, the scientists will return to the city with their reams of binary code, leaving the engineers alone in the mountains once more.

Yet the team has to work extremely hard to make the researchers’ work possible. Lei, who has worked at FAST for eight years while completing a Ph.D. with the Chinese Academy of Sciences, says he often spends hours fixing malfunctioning parts.

“It’s super annoying,” he says. “When I finally get to organize my thoughts and start writing my thesis, a call comes in about a broken device. It’s not like I can ignore them.”

Lei’s job is to look after the actuators, the hydraulic winches that manipulate the telescope’s dish so it can focus waves from a certain part of the cosmos onto the antenna.

There are over 2,000 actuators, and they all have to remain accurate to within 0.25 millimeters for FAST to function properly. When one breaks down, it can take four engineers up to three hours to fix.

“These devices can be troublemakers here,” Lei tells Sixth Tone, as he drives his white pickup truck along the narrow path leading to the base of the dish. “We’re checking actuators every day, to make sure they work properly.”

Wu Ruofei, a 36-year-old who is responsible for maintaining the six suspension cables and support towers, has also become accustomed to things going wrong.

Like many of his teammates, Wu arrived at FAST in 2013, when construction of the telescope was just beginning. At the time, the site looked wild and unkempt, with only verdant mountains visible for miles around. Even the road up to the crater hadn’t been completed yet.

Now, he spends much of his time clambering over the telescope, checking if the pulleys are functioning correctly, looking for rust on the towers, and ensuring the machinery is well-lubricated.

The work is usually routine, but Wu sometimes finds unexpected guests. Snakes lurk in the bushes under the dish, while mice chew on the wires. The biggest problems, however, come when wasps build nests on the support towers.

On one occasion, an insect attack forced a group of engineers to flee their tower, Wu says. They raced down the 30-story structure in under five minutes, but still received serious injuries.

“They ended up in the hospital — wasps are no joke,” says Wu. “We now keep bottles of pesticide spray here and there at the site, just in case.”

The engineers acknowledge there are many downsides to their work. The heat and humidity on the mountains are intense, and the mosquitoes are relentless. The site, moreover, is almost entirely cut off from the outside world — the only place with internet and mobile access is a four-story building around a kilometer from the dish, which houses FAST’s control room and staff accommodation. Wu’s family lives in the city, a two-hour drive away. He only sees them once every eight days.

But there are compensations. Leaning against the railing at the top of one of the towers, Wu says he’d rather be doing this than working in an office any day. He looks relaxed and fit, his skin tanned to a deep brown.

For Lei, it’s something more intangible that keeps him in the mountains. Having watched the telescope being built from scratch, maintaining it feels like more than a job to him.

“Taking care of it is like doing housework for me,” he says. “It’s just normal stuff you ought to do.” He pauses, then adds: “It’s something that’s unlike any other work.”

A few days after Sixth Tone’s visit, the team finally manages to finish replacing the cables. After the first two, the team grew familiar with the procedure and was able to finish the job very quickly, says Yao, the project coordinator. By the end of July, FAST returned to normal working order, 10 days ahead of schedule.

It’s a huge relief to Yao. She has now returned to headquarters in Beijing, where her family is based. She’ll return to Guizhou if another maintenance project needs to be handled, she says.

For the engineers, the hope is that the telescope can now go on to make more exciting discoveries. In January, FAST passed a government inspection and was declared fully operational, its multiyear trial period officially over.

Currently, FAST is only open to projects led by domestic scientists, though many international astronomers have expressed strong interest in working with it. The engineers say all the observation data they’ve collected will become publicly available in the future, in line with international norms in the astronomy community.

For the rest of the year, the telescope will run at full capacity — 24 hours a day, seven days a week — tuning into signals from deep space.

Wu’s dream is for FAST to discover a previously unknown celestial body — he doesn’t know what specifically, just something new, he says.

“My son once said at his kindergarten that his dad is a scientist,” says Wu with a grin. “But I’m really just someone working for scientists. If they get a Nobel Prize using FAST, that’ll be the closest I ever get to winning one.”

The facility is designed to operate for at least the next 30 years, though several engineers tell Sixth Tone they’re confident the telescope will be capable of continuing well beyond its “best before” date.

Yao, however, accepts FAST will one day no longer be needed. It’s inevitable that new technologies will one day make the facility redundant, just as FAST has supplanted the Arecibo Observatory in Puerto Rico, she says.

“As long as FAST keeps making discoveries, that’s enough to keep me happy,” says Yao. “That means the telescope is working well, and that means a lot to me.”