What followed has often been described as the most nerve-racking elevator ride in the history of exploration.

The Mission

On 23 January 1960, the Trieste began its descent into a part of the ocean humans had never seen. The target depth was nearly 11,000 meters (about 36,000 feet)—far deeper than the height of Mount Everest above sea level.

At those depths, conditions are so extreme that almost all conventional engineering solutions fail. Pressure rises to more than 1,000 atmospheres, darkness is absolute, and temperatures hover just above freezing.

The descent alone took almost five hours.

Engineering the Impossible

The Strange Float

The Trieste did not look like a typical submarine. Instead of a streamlined military hull, it resembled a massive floating tank with a small steel sphere suspended beneath it.

Modern submarines adjust buoyancy using air ballast tanks. That approach would have failed catastrophically in the deep ocean. At nearly seven miles down, air would compress so much it would lose most of its lifting power.

The solution was unconventional: 22,000 gallons of gasoline.

Gasoline is lighter than seawater and, unlike gases, it remains almost incompressible under extreme pressure. This allowed the Trieste to maintain buoyancy regardless of depth. The fuel was stored inside a giant float chamber that formed the bulk of the craft.

Engineers also took advantage of another property: gasoline freezes at a much lower temperature than water, meaning it would remain usable in near-freezing abyssal conditions.

The Descent System

To descend, the bathyscaphe relied on gravity.

Approximately nine tons of iron shot pellets were stored in hoppers and held in place by electromagnets. This design created one of the most elegant safety mechanisms ever used in exploration engineering.

If anything went wrong—power failure, electrical malfunction, or onboard emergency—the electromagnets would deactivate automatically.

The pellets would fall away.

The sub would become instantly lighter than the surrounding water and float back to the surface.

In other words, the craft had a built-in fail-safe that required no human decision.

The Pressure Problem

The most dangerous challenge was pressure.

At the bottom of the Mariana Trench the water pressure reaches roughly 16,000 pounds per square inch (about 110 MPa). That is more than 1,000 times the pressure at sea level.

To survive it, the crew sat inside a steel sphere just over two meters wide, with walls more than 5 inches (13 cm) thick.

Spheres are ideal for deep pressure because the stress distributes evenly in all directions. Even so, every bolt, seal, and window represented a potential point of failure.

A structural collapse at that depth would occur instantly.

There would be no warning.

The Moment Everything Nearly Failed

About 30,000 feet into the descent, disaster nearly struck.

A loud bang, like a rifle shot, echoed through the cabin.

One of the outer plexiglass viewports had cracked under the intense cold and pressure.

For a few seconds, Piccard and Walsh had no way of knowing whether the damage would cascade into catastrophic failure. Outside their tiny sphere sat millions of tons of seawater pressing inward with unimaginable force.

But something remarkable happened.

Instead of blowing inward, the pressure forced the remaining layers of the viewport tighter into the frame, sealing it even more firmly.

The crack looked terrifying.

But structurally, the window held.

The descent continued.

Arrival at the Bottom

After nearly five hours, the Trieste reached the seabed of the Challenger Deep. A cloud of silt billowed upward as the craft settled. Outside the viewport stretched a silent, pale landscape that no human eyes had ever seen before.

Contrary to expectations, the pilots reported signs of life—small organisms moving across the seafloor, suggesting that life could exist even in the most extreme parts of Earth’s oceans.

They remained on the bottom for about 20 minutes.

Long enough to confirm depth readings.
Long enough to look around.
Long enough to realize they had reached the deepest place on the planet.

The Return

To ascend, they released the iron ballast.

Tons of metal rained down into the trench, instantly making the Trieste positively buoyant.

The sub began its long climb back toward sunlight.

Several hours later, the craft broke the surface of the Pacific Ocean, completing one of the boldest engineering and exploration feats in human history.

For decades afterward, no one returned to that depth.

Why the Dive Still Matters

The Trieste dive remains one of the greatest demonstrations of engineering simplicity meeting extreme environments:

• Passive safety systems
• Pressure-optimized geometry
• Clever use of material properties
• Redundant buoyancy logic

Even modern deep-sea vehicles still rely on many of the same physical principles.

In an era before advanced composites, computer modeling, or digital monitoring, two men descended nearly 11 kilometers into the Earth in what was essentially a steel ball hanging under a gasoline tank.

And somehow, it worked.

Watch below the video documentary: