Imagine sitting on a narrow bench inside a dark room. Your feet are dangling into a floor of water. You’re vaguely aware of the room moving. Your ears start ringing. If you move too much, you feel the room sway, which could bring the floor rushing in to fill it. You take a breath and dive down, swim outside the room, groping the water, looking for its bottom, reaching for something valuable enough to take back with you.

If you’ve ever pushed an upside-down cup into water, reached inside, and found it still empty, you’ve encountered a diving bell. It’s a simple concept: The water’s pressure forces the air, which has nowhere else to go, inside the “bell.” Once people realized that trapped air contains breathable oxygen, they took large pots, stuck their heads inside, and jumped into the nearest body of water. In the 2,500 years since, the device has been refined and expanded to allow better access to the ocean’s depths. But that access has not come without human cost.

* * *

The first account of diving bells comes from Aristotle in the 4th century B.C.E. Legend has it Aristotle’s pupil Alexander the Great went on to build “a very fine barrel made entirely of white glass” and used it in the Siege of Tyre in 332 B.C.E. However, the facts of Alexander the Great’s adventures come mostly from depictions in fragments of ancient art and literature, which render him as a demigod who conquered the darkness and returned to the dry realm of historians and poets.

Prior to the diving bell, the wet parts of the earth were places people could move atop but not transit within. Shallow diving was possible: Duck hunters in ancient Egypt and swimmers in Rome and Greece used hollowed-out reeds or plant stems as snorkels. But they were still surface-bound, barely deeper than the reflections of the sky on the water above.

The diving bell changed that. Figuring out how to stay underwater was a turning point in not only naval technology, but also in science and adaptation. The diving bell acted as a portable atmosphere, allowing divers to descend a dozen feet or so, briefly leave the bell, return to it for air, and then return to the surface and start all over once they had filled their home base with too much carbon dioxide.

Staying submerged began as a simple trick, a novelty meant mostly for spectacle. But like most human exploration, the underwater landscape became appealing for its latent revenue opportunities. At first, diving bells appear to have been most heavily used in the pearl and sponge industries. Then, in 1531, the Italian inventor Guglielmo de Lorena came up with a new application. Using slings to attach a bell to his body, he could collect treasure from capsized Roman ships. After the defeat of the Spanish Armada in 1588, according to Francis Bacon, Spanish prisoners spread the word that their captors’ riches had sunk off the coast of Scotland; industrious divers used bells to pick up the scraps.

Seeing the technology as a business opportunity, scientists and inventors made improvements to a concept that had shown virtually no change in two millennia. A renaissance era for the diving apparatus commenced. The German painter and alchemist Franz Kessler, the Swedish colonel Hans Albrecht von Treilebe, the Massachusetts Bay Colony governor Sir William Phips (best known today for the Salem witch trials), the French priest Abbe Jean de Hautefeuille, the French physicist Dennis Papin, and the British super-scientist Edmund Halley (of Halley’s Comet fame) all made contributions to diving bell technology in the 17th century—all in the interest of collecting valuables no one else could reach. Phips went as far as modern-day Haiti to chase sunken treasure.

The most important of these contributions may have come from de Hautefeuille, who in 1681 wrote that diving deeper alters the atmospheric pressure of the air available to a diver. Pressure was the key to more sustainable expeditions, it turned out. Halley then developed a complex system of weighted air barrels, hoses, and valves to keep a relatively stable level of oxygen and pressure inside his lead-reinforced wooden bell design.

But increasing the pressure inside the bell posed a problem. It also kept the water level down as the bell descended, and thereby pressurized the bell’s inhabitants, occasionally bursting divers’ ear drums. Using faucets to adjust the pressure inside the bell and sending barrels back-and-forth to the surface to replenish his air supply, Halley was able to spend well over an hour 60 feet below the surface, though he did complain that his ears felt “as if a quill had been thrust into them.”

Casualties became a theme. In 1775, the Scottish confectioner Charles Spalding improved diving-bell safety with better balance weights, a pulley system that increased dive control, signal ropes leading to the surface, and even windows. Spalding and his nephew, Ebenezer Watson, used such diving bells for salvage work—until they both suffocated inside one off the coast of Ireland.

The final contribution to the Halley-style “wet” (partially-enclosed) bell was by Englishman John Smeaton more than a decade later. Smeaton’s bell maintained the air supply by connecting a hose to a pump above the surface. This design enabled laborers to fix the foundation of England’s Hexham Bridge. But it led to lower-class caisson workers coming down with what they called “caisson sickness,” as Smeaton’s bell became ubiquitous in harbors throughout the world. Now known as the bends or decompression sickness, caisson sickness sometimes caused surfaced divers to have strokes, leading to paralysis and even death. Workers would come back to the dry world and pray they didn’t mysteriously take ill.

It wouldn’t be until nearly 1900 that scientists began to master the effects of pressure on the human body. Eventually, the wet bell gave way to modern, completely enclosed “dry” bells, which were really just pressurized diving chambers. By the mid-20th century, these sophisticated diving bells aided the booming offshore oil industry—fuel awaited human discovery in the deep, next to shipwrecks, sponges, and pearls.

* * *

Alexander the Great reportedly claimed that while submerged, he saw a monster so massive it took it three full days to swim past him. It would have been physically impossible to survive in his bell that long, of course, but the story makes good legend: The ocean offers a void big enough to contain human metaphor and myth, an emptiness vast enough to consume a three-day-long behemoth—or to swallow the continent of Atlantis (as Plato claimed), or the whole Earth itself (as Noah’s God commanded), or to hide the Missing Link on the lost island of Lemuria, or to conceal countless missing vessels in the Bermuda Triangle.

It’s no coincidence that the psychoanalyst Carl Jung chose Proteus, the shape-shifting Greek god of water who tells the future to whoever can catch him, as a manifestation of the unconscious, that great dark sea in the mind. The ocean represents the unknown. For thousands of years, it marked the portion of Earth people could never access. It was a place conquerable only by God, whom Isaiah addressed, “Art thou not it that dried up the sea, the waters of the great deep; that made the depths of the sea a way for the redeemed to pass over?”

Imagine again: Your arms are full of something heavy you imagine to be precious, and you spend the last of your breath kicking, ascending back to the small room where you hope there’s still enough air to breathe, and then enough to make it to the surface, where you’ll wait to find out whether you’ll become one of the sick ones. Someone will pay you, and maybe it’ll be enough. To go underwater always challenges humanity’s natural place; to strive to stay there is to defy our given position on the earth. But humans will persist, because still so little is known about what lurks deep in the ocean, and because discovering it is worth the trial of pursuit.

This article appears courtesy of Object Lessons.