In 1960, Captain Don Walsh, a 28-year-old submarine lieutenant, embarked on a historic journey to descend deeper into the ocean than any human had gone before. The US Navy had acquired the bathyscaphe Trieste, and Don volunteered to join the project despite having only been 100 meters down in a submersible before. The mission's goal was to reach the deepest place on Earth—the bottom of the Mariana Trench, a narrow underwater canyon in the Pacific Ocean.

On January 23, 1960, Don Walsh and Swiss oceanographer Jacques Piccard, who designed the bathyscaphe with his father Auguste Piccard, began their descent. Inside the small steel-walled chamber, the size of a large household refrigerator, they gradually descended into the darkness. The pressure increased, and the bathyscaphe, designed to withstand extreme pressures, began to creak and groan.

At around 9,000 meters, a jolt and a bang startled them, but after checking the instruments and finding everything in order, they continued. After five hours, they passed 10,000 meters, still without sighting the sea floor. Finally, at almost 11 kilometres (seven miles) down, the glow of the bathyscaphe's lights reflected back towards them—they had reached the bottom of the Mariana Trench.

The descent was not without challenges. Upon inspecting the craft, they discovered a cracked acrylic window inside the entrance hatch, a potentially catastrophic issue. Nevertheless, they spent about 20 minutes on the seafloor before safely returning to the surface.

The dive into the Mariana Trench became a defining moment for Don Walsh, and he spent the rest of his life advocating for ocean exploration and safety within the industry. He warned of potential disasters, such as the Titan tragedy, and continued his involvement in deep-sea exploration.

In 2012, Hollywood filmmaker and ocean explorer James Cameron repeated the dive, and in 2019, Victor Vescovo explored the trench with Don's son, Kelly Walsh, on board.

Don's legacy extends beyond his pioneering dive. Recent technological advances have revealed the significance of the ocean trenches in Earth's systems, including their role in the carbon cycle and climate change. The exploration sparked by Don and Jacques Piccard's historic feat continues to shed light on the mysteries of the deep sea.