A few years before Norm Pace revolutionized the study of life on Earth, he almost lost his own life.

He’s best known as a microbiologist, but he’s also a die-hard spelunker. In the 1970s, he and a group of friends set out to explore a Mexican cave called Sumidero Yochib. It sits at the bottom of a massive sinkhole that doubles as a large drain; when it rains, a hundred square miles of water funnel into the entrance, turning it into a raging torrent. These waterfalls, combined with almost a dozen sheer drops, make Yochib one of the most dangerous caves in the world.

Pace learned that the hard way. During one of his expeditions into Yochib, the bolt that was holding Pace’s rope popped out. He fell, straight into the path of a waterfall. His companions tried to pull him up, but failed. They headed back to the surface, sure that they had just seen their friend die. But Pace was very much alive. He had used his helmet to create an air pocket, which allowed him to breathe amid the cascading water. Slowly, he climbed his way out. His friends were trying to work out how to recover his body, when said body walked up to them and calmly sat down.

Norm Pace has a wiry build, large round glasses, and a deep voice that is already resonant at ground level, and one imagines even more so within the caves he likes to explore. He first entered a cave when he was 14 years old. Now, at the age of 75, he has explored more than 100 of them. In 1987, he received the Lew Bicking Award—the highest honor that American cave explorers can hope to get. “Have you ever walked down a sand passage that was deposited in the Pleistocene, and you’re putting the first footprints in it?” he asks me, in his deep, resonating voice. “Have you ever dropped down a waterfall, and no one knows where it goes? There’s this whole unknown world out there that we didn’t even know existed.”

The same could be said about Pace’s other passion—life on Earth. Most of it is microscopic and, until recently, most of it was completely unknown. To understand life on Earth, we need to first answer this simple question: What lives where? This is the essence of biogeography. It’s the question that sent naturalist-explorers like Charles Darwin and Alfred Russel Wallace sailing around the world, cataloguing its species, and eventually gaining the inspiration for the theory of evolution. It’s also a question that microbiologists had long ignored. The feeling was that “we already knew what was out there,” Pace once told me, when I interviewed him for my recent book. “People went out, overturned a rock, found a bacterium and thought it exemplary of what’s out there. It was stupid.”

When Pace started his career, scientists who studied bacteria and other microbes did so by growing them in flasks and beakers, but only a tiny fraction of these organisms will thrive in a laboratory. “When I started as a microbiologist, [we] just accepted that we had no idea of what 99.9 percent of microbes in the environment did, and focused our energies on working out what the 0.1 percent did,” says Hazel Barton, a microbiologist at the University of Akron and one of Pace’s protégés.

But in the 1980s, Pace pioneered ways of identifying microbes without having to grow them at all. Instead, he just pulled their genes out of the environment and sequenced them. That changed everything, allowing scientists to fully grasp just how many microbes were out there—in the soil, in water, in our very bodies. The science of the human microbiome—the community of microbes that live in and on us—is one of the hottest fields of biology right now. And in the words of the microbiologist and former postdoctoral researcher of Pace’s, Ruth Ley, “There wouldn’t be the microbiome without Norm Pace.”

“I don’t expect an epitaph,” Pace says, “but if I had one, I’d want it to be: ‘He blew the door off the natural microbial world.’ What more can one say?”

“It’s a still, beautiful, and lethal place. I told the students: Don’t fall in.”

“I was always fascinated with the hidden world,” Pace once said. Growing up in rural Indiana, he spent a lot of his childhood gazing at objects through a toy microscope, while playing around in the fairly sophisticated lab that he had set up in his basement. (“Those dabblings lead one into rocketry and other things that aren’t good for you,” he tells me, in a cryptic hint at what sorts of experiments he might have gotten up to down in that basement.)

A few decades later, in the early 1970s, Pace had a proper lab of his own at the National Jewish Hospital and Research Center. There, he focused on understanding molecules called rRNA, which form the protein-making machinery that all living things depend upon. Pace’s friend Carl Woese had recently shown that one could work out the relationships between living things by comparing the subtly different versions of rRNA in their cells, rather than just by looking at their anatomy or other physical traits. This revolutionary approach set the course for much of modern biology. It also revealed the existence of the archaea—a group of microbes that had been wrongly mistaken for bacteria, but are actually a separate domain of life.

Many archaea live in extreme environments that other microbes cannot withstand. In the 1980s, Pace wanted to study the rRNA of these extremophiles, but he faced the same problem that plagued microbiologists for centuries: “The organisms were a pain in the ass to grow,” he recalls. He just couldn’t get enough rRNA out of them. Then, he read about Octopus Spring—a natural cauldron in Yellowstone National Park, whose roiling, deep blue water can reach 196 degrees Fahrenheit. In that water, archaea grow in such huge swarms that they manifest as visible pink filaments. Here was all the rRNA Pace needed, no culturing necessary. Pace remembers rushing out to his lab and shouting, “Hey, guys, look at this! Kilogram quantities! Let’s get a bucket and go up there.” One of his team said, “But you won’t even know what the organism is.”

And Pace replied: “That’s okay. We can sequence for it.”

It was a bona fide eureka moment, and one whose importance is hard to overstate. Pace had finally given biologists a way of studying the “uncultured majority”— the microbes that refuse to grow in laboratories. You didn’t need to grow them, or even to see them. You could just pull their RNA, or their DNA, right out of the environment, sequence the lot, and work out who was where. “We took our bucket up to Yellowstone and did it,” he says. “It’s a still, beautiful, and lethal place. I told the students: Don’t fall in.”

From Octopus Spring, Pace and his team identified two bacteria and an archaeon—all new to science. Their results, published in 1984, marked the first time that anyone had discovered an organism from its molecules alone. Several years later, Pace and his student Ed DeLong analyzed samples of plankton that they had fished out of the Pacific Ocean. Again they found a community of unknown bacteria—15 new species, of which two were only distantly related to any known group.

At the time, all known bacteria fit into a dozen major groups, or phyla. By 1998, that number had blossomed to around 40. When I spoke to Pace in the summer of 2015, he told me that we were up to 100. A month later, Jill Banfield from the University of California, Berkeley announced the discovery of around 35 more. Most of these groups have never been cultured before. Most have never been seen under a microscope. Their existence is known solely because of their genes. Slowly but surely, microbiologists have started to fill out the branches, twigs, and leaves of the microbial tree of life.

“We haven’t begun to touch the golden age of microbiology. It’s ahead of us.”

Pace and his colleagues have also helped to work out what all these unknown microbes actually do. They developed ways of sequencing not just rRNA, but every microbial gene in a sample. That told them not just who was present, but what these microbes were capable of.

“If you look at textbooks, it’s not uncommon for the historical section to say that the golden age of microbiology ran from 1900 to 1950, when new antibiotics were discovered,” says Pace. “We haven’t begun to touch the golden age of microbiology. It’s ahead of us.” Since his seminal work, Pace has studied the microbes of many other inhospitable environments, including caves, hydrothermal vents at the bottom of the ocean, and the New York subway. He has also turned his attention to more everyday ecosystems, like showerheads and offices. (One of his regular talks is called “In Your Face: The Microbiology of Indoor Waters.”)

Beyond just identifying microbes himself, Pace has trained many of the big names in the microbiome field. Many of them recently gathered in Colorado for the “Normposium” conference to commemorate Pace’s retirement, and Pace presumably greeted them all with friendly fistbumps—he avoids shaking hands where possible.

His colleagues describe him as irreverent, boyishly enthusiastic, and intimidatingly smart. Ed DeLong remembers telling him everything that he had been working on for the previous month. “I stepped out of the lab to grab a cup of coffee and was gone for about 10 minutes,” DeLong says. “As I passed Norm’s office on my return, he saw me go by and said: ‘So, Ed, what’s new?’ I had to tell him, ‘Norm! I got a cup of coffee!’”

Pace also has a reputation for being straight-talking, to the point of rudeness if he thinks someone is wrong. “He’s always been very direct and unvarnished: What you see is what you get,” says Philip Hugenholtz, a microbiologist and former postdoc of Pace’s. He has a somber side to him, too. When I last spoke to him, he spent a lot of time talking about population growth and doomsday situations—a tendency that others who know him also recall. “It was like the weather with Norm: You were never quite sure what it was going to be,” says Ruth Ley. “He would bring up these gloomy scenarios of mass extinction, and he once told me: Don’t blink because your life’s over before you know it.”

In March, a few days before he was due to leave for a conference, Pace experienced a seizure and some strokes and went to hospital for brain surgery. That was how he learned that he has advanced melanoma, which has spread to other parts of his body. He’s now in the process of being treated with radiotherapy, and treatments that are meant to marshal his immune system against the cancer. His ex-wife Bernadette—another microbiologist and professional trapeze artist—flew over to take care of him, and after about 20 years apart, the couple re-married in May.

When I spoke to him in June, Pace was lucid, thoughtful, and realistic about his prognosis. He quoted the opening verse of Samuel Taylor Coleridge’s Kubla Khan to me.

In Xanadu did Kubla Khan A stately pleasure-dome decree: Where Alph, the sacred river, ran Through caverns measureless to man Down to a sunless sea.

“My background in caving,” he said, “leads me down to a sunless sea.”