Louis Pasteur was at his most comfortable when working in his Paris laboratory. It was there that he had some of his greatest scientific triumphs, including experiments that helped confirm germs can cause disease. “Everything gets complicated away from the laboratory,” he once complained to a friend.

But in 1860, years before he became famous for developing vaccines and heating milk to kill pathogens, Pasteur ventured to the top of a glacier, on a remarkable quest for invisible life.

He and a guide began at the base of Mont Blanc in the Alps, hiking through dark stands of pines. Behind them, a mule carried baskets of long‑necked glass chambers that sloshed with broth. They ascended a steep trail until they reached Mer de Glace, the sea of ice.

The wind blew briskly over the glacier, and the vale echoed with the sound of frozen boulders crashing down the slopes. Pasteur struggled to make out the path in the glare of sunlight bouncing off the ice.

When the scientist reached an altitude of 2,000 meters, he finally stopped. He removed one of the glass chambers from the mule’s pack and raised it over his head. With his free hand, he grabbed a pair of tongs and used them to snap off the end of the neck. The cold air rushed inside the container.

The sight of Pasteur holding a globe of broth over his head would have baffled other travelers visiting Mer de Glace that day. If they had asked him what he was doing, his answer might have seemed mad. Pasteur was on a hunt, he later wrote, for “the floating germs of the air.”

Now, 165 years later, scientists around the world hunt for floating germs. Some study how coronaviruses wafting through buses and restaurants spread Covid. Spores of fungi can travel thousands of miles, infecting people and plants. Oceans deliver microbes into the air with every crashing wave. Even clouds, scientists now recognize, are alive with microbes.

The sky’s ecosystem is known as the aerobiome. In Pasteur’s day, it had no name. The very idea of living things drifting through the air was too strange to imagine.

But Pasteur began to wonder about the possibility of airborne life when he was a little-known chemist teaching at the University of Lille in France. There, the father of one his students approached him for help. The man owned a distillery where he used yeast to turn beet juice into alcohol. But the juice had inexplicably turned rancid.

Inspecting the liquid under a microscope, Pasteur discovered dark rods — bacteria rather than yeast — in the sour vats. The discovery helped him work out a theory of fermentation: Microorganisms absorbed nutrients and then produced new compounds. Depending on the species, they could turn butter rancid or grape juice into wine.

The discovery won Pasteur a prestigious new post in Paris. In his account of the discovery, Pasteur suggested in passing that the bacteria might have floated through the air and settled into the vats. That notion earned him an angry letter from Félix‑Archimède Pouchet, one of France’s leading naturalists.

Pouchet informed Pasteur that the microorganisms Pasteur discovered had not dropped into the vats from the air. Instead, the beet juice had spontaneously generated them. “Spontaneous generation is the production of a new organized being that lacks parents and all of whose primordial elements have been drawn from ambient matter,” Pouchet had written earlier.

Pasteur coolly replied that Pouchet’s spontaneous generation experiments were fatally flawed. The conflict between Pasteur and Pouchet prompted the French Academy of Sciences to announce a contest for the best study addressing whether spontaneous generation was real or not. What started as a private spat had turned into a public spectacle. Pasteur and Pouchet both signed up to compete for the prize of 2,500 francs.

The public eagerly followed the competition, struggling to imagine either view of life. Spontaneous generation had the whiff of blasphemy: If life could spring into existence, it did not require divine intervention. But Pasteur’s claim that the atmosphere teemed with germs also strained the 19th‑century mind. A French journalist informed Pasteur that he was going to lose the contest. “The world into which you wish to take us is really too fantastic,” he said.

To prove that his world was real, Pasteur set out to pluck germs from the air. Working with glassblowers, he created flasks with narrow openings that stretched for several inches. He filled them with sterile broth and waited to see if anything would grow inside. If the necks were pointed straight up, the broth often turned cloudy with microorganisms. But if he sloped the necks so that the openings pointed down, the broth stayed clear. Pasteur argued that germs in the air could drift down into the flasks, but could not propel themselves up a rising path.

When Pouchet heard about Pasteur’s experiments, he sneered. Did Pasteur really believe that every germ in decaying organic matter came from the air? If that were true, every cubic millimeter of air would have been packed with more germs than all the people on Earth. “The air in which we live would almost have the density of iron,” Pouchet said.

Pasteur responded by changing his hypothesis. Germs were not everywhere, he said. Instead, they drifted in clouds that were more common in some places than others.

To prove his claim, Pasteur took his straight-necked flasks out of his lab and began collecting germs. In the courtyard of the Paris Observatory, all 11 of his flasks turned cloudy with multiplying germs. But when he traveled to the countryside and ran his experiment again, more of his flasks stayed sterile. The farther Pasteur got from human settlements, the sparser airborne life became. To put that idea to an extreme test, Pasteur decided to climb Mer de Glace.

His first foray to the glacier ended in failure. After holding up a flask, he tried using the flame from a lamp to seal its neck shut, but the glare of the sun made the flame invisible. As Pasteur fumbled with the lamp, he worried that he might be contaminating the broth with germs he carried on his skin or his tools. He gave up and trudged to a tiny mountain lodge for the night.

He left his flasks open as he slept. In the morning they were rife with microorganisms. Pasteur concluded that the lodge was packed with airborne germs that travelers had brought from around the world.

Later that day, Pasteur modified his lamp so that the flame would burn bright enough for him to see it under the glacier-reflected sun. When he climbed back up Mer de Glace, the experiment worked flawlessly. Only one of the flasks turned cloudy with germs. The other 19 remained sterile.

In November 1860, Pasteur arrived at the Academy of Sciences in Paris with the 73 flasks he had used on his travels. He entered the domed auditorium, walked up to the table where the prize committee sat, and laid out the flasks. The judges peered at the broth as Pasteur described his evidence, saying it gave “indubitable proof” of floating germs in inhabited places.

Pouchet refused to accept the evidence, but nevertheless withdrew from the contest. Pasteur was awarded the prize.

Still, the two continued to spar. The rivalry remained so intense that the Academy set up a new commission to evaluate their latest experiments. Pouchet dragged out the proceedings, demanding more time for his research.

Pasteur decided to seize public opinion and put on a spectacle. On the evening of April 7, 1864, in an amphitheater filled with Parisian elites, Pasteur stood surrounded by lab equipment and a lamp to project images on a screen. He told the audience it would not leave the soiree without recognizing that the air was rife with invisible germs. “We can’t see them now, for the same reason that, in broad daylight, we can’t see the stars,” he said.

At Pasteur’s command, the lights went out, save for a cone of light that revealed floating motes of dust. Pasteur asked the audience to picture a rain of dust falling on every surface in the amphitheater. That dust, he said, was alive.

Pasteur then used a pump to drive air through a sterile piece of cotton. After soaking the cotton in water, he put a drop under a microscope. He projected its image on a screen for the audience to see. Alongside soot and bits of plaster, they could make out squirming corpuscles. “These, gentlemen, are the germs of microscopic beings,” Pasteur said.

Germs were everywhere in the air, he said — kicked up in dust, taking flights of unknown distances and then settling back to the ground, where they worked their magic of fermentation. Germs broke down “everything on the surface of this globe which once had life, in the general economy of creation,” Pasteur said.

“This role is immense, marvelous, positively moving,” he added.

The lecture ended with a standing ovation. Pasteur’s hunt for floating germs elevated him to the highest ranks of French science.

By the time he died 31 years later, Pasteur had made so many world-changing discoveries that his many eulogies and obituaries did not mention his trip to Mer de Glace.

But scientists today recognize that Pasteur got the first glimpse of a world that they are only starting to understand. They now know that life infuses the atmosphere far more than he had imagined, all the way to the stratosphere. Our thriving aerobiome has led some scientists to argue that alien aerobiomes may float in the clouds of other planets. Ours is not the only world that seems too fantastic to believe.