What do you do when you want to go to the lowest point on the surface of the Earth—a place so deep beneath the ocean it could crush you with its intense pressure? If you’re Swiss scientist Auguste Piccard, you build a bathyscaphe, of course.

The object above is Trieste, the first-ever craft to make it all the way to the Challenger Deep, the lowest place in the Mariana Trench (and thus the entire ocean), in 1960. The craft was designed by Piccard, an adventurous physicist, inventor, and explorer who had previously been known for his daring expeditions into the sky. In 1931, he had ascended almost 10 miles into the atmosphere in an airtight aluminum ball tucked into a hot air balloon, demolishing aircraft altitude records and making valuable observations about the behavior of cosmic rays.

But Piccard didn’t just want to go upward. He was also obsessed with going in the other direction: down into the oceans. To make such a feat possible, he invented the bathyscaphe, a kind of inverse of his hot air balloon ball. The concept—a self-propelled, submersible diving vessel—was an improvement on the bathysphere, a kind of deep-sea bubble lowered to the ocean with a cable, which had been invented by Americans William Beebe and Otis Barton in the late 1920s.

The pressure at the bottom of the ocean is so great it can crush submarines, not to mention lesser craft. To resist that pressure, the Trieste relied on a heavy steel crew cabin, as well as separate tanks filled with gasoline and air. The gasoline—which is lighter than water and does not compress under pressure like some other substances—helped the crew to maneuver and navigate. The air tanks, which would slowly fill with water while descending, helped the vessel to descend, and worked in concert with a system of cone-shaped containers filled with iron ballast. To ascend back up to the surface, magnets would release the iron ballast.

Piccard built his first bathyscaphes in the 1940s and 1950s, but the Trieste was the most ambitious of them all. The inventor supervised its building for the French Navy, which used it for several years. In 1958 the U.S. Office of Naval Research bought it for its riskiest trip yet—a descent to the world’s deepest place, the Mariana Trench.

Piccard, however, was in his seventies, and did not go along for the trip. He sent his son Jacques instead, along with an American Navy lieutenant named Don Walsh. Before completing Project Nekton, as it was called, the group did multiple test dives in Guam. Then the fateful day came: January 23, 1960. The hydronauts equipped themselves with chocolate bars and sonar hydrophones and headed down … and down … and down.

So what was there to see so far down in the ocean? Some pretty weird stuff, it turns out: sediment the hydronauts described as “diatomaceous ooze,” and bioluminescent creatures gleaming against the darkness. It took five hours to get the seven miles down and another three to get back up, but by the time Piccard and Walsh emerged, exhausted, they were heroes.

For years, nobody ever returned to the Challenger Deep, not until James Cameron managed a much-hyped solo dive there in 2012. But Piccard and Walsh were the first—and these days, the craft that took them to that mysterious place lives in the National Museum of the United States Navy in Washington, D.C. True to its famous form, it’s the museum’s most photographed artifact, and a reminder that sometimes the race to the bottom can be a good thing.