The vehicle used for the dive is named Limiting Factor.

©Triton Submarines; Shutterstock.com (badge)

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Journey to the Bottom of the World

An underwater explorer journeys to the ocean’s deepest point.

January 3, 2022
by Lisa M. Herrington

In 2020, Vanessa O’Brien crammed into a small deep-sea vehicle with pilot Victor Vescovo. She was headed into the unknown.

Eight years earlier, O’Brien had climbed to the top of Mount Everest. That’s the world’s highest point. Now, she was trying to reach its lowest: Challenger Deep. It’s the deepest part of the Mariana Trench, a long, narrow canyon in the Pacific Ocean.

O’Brien and Vescovo’s vehicle is called a submersible. It was lowered into the water by a crane from a ship. Within minutes, the deep blue of the ocean turned to pitch-black. As the sub slowly descended, the water got darker and colder. It was completely quiet. After about four hours, Vescovo said, “Touchdown!”

They’d finally reached the deepest place on the planet.

“The bottom looks a little bit like what I imagine the moon to look like,” says O’Brien. “It’s very still. This is an uninterrupted, unvisited place.”

©Enrique Alvarez 2020

Vanessa O’Brien and Victor Vescovo inside their submersible

Mysteries of the Deep

The ocean covers about 70 percent of Earth’s surface, but humans have yet to explore most of it. In fact, less than 20 percent of the seafloor has been mapped.

The biggest enemy of ocean explorers is water pressure—the force of the water pressing on their bodies as they go deeper into the ocean. Most scuba divers can’t go much deeper than a few hundred feet. At greater depths, the pressure can easily crush a person and most submarines.

To withstand the enormous pressure, Vescovo had his sub built with one of the strongest metals on Earth. In 2019, he became just the fourth person to ever reach Challenger Deep. A year later, O’Brien became the ninth.

Exploring the Seafloor

Jim McMahon/Mapman®

The Mariana Trench is about five times longer than the Grand Canyon. During their expedition, O’Brien and Vescovo mapped a mile-long stretch of the trench. They also collected water samples and took photos and videos.

The pressure, darkness, and cold temperature make it difficult for most sea creatures to survive at that depth. But some life exists there. The sub’s cameras showed tiny creatures called bristle worms along the sandy trench floor.

After three hours, the two adventurers began the long, slow journey back to the surface.

To the Extreme

With this mission, O’Brien joined Vescovo as the only two people to reach Earth’s highest and lowest points. O’Brien says Everest and Challenger Deep have more in common than most people think. She points out that both are cold, lack oxygen, and aren’t natural environments for humans to live in.

But, O’Brien says, you don’t need to go to the world’s most extreme places to explore.

“All kids have to do is tap into their curiosity and go and explore with an open mind,” she explains.

Layers of the Ocean

Scientists divide the ocean into five zones, or layers. Conditions such as temperature and water pressure change from zone to zone. Sunlight reaches only the top two zones, where most ocean animals live.

Note: This diagram does not represent a single location. Animals are not shown to scale. Most figures are rounded.

Oceangate Expeditions (Titanic); Courtesy Oceanlab, University of Aberdeen (snailfish); Shutterstock.com (all other images)
  • 0 FEET DEEP: SUNLIGHT ZONE
    • 1,090 feet: Deepest scuba dive
  • 660 FEET DEEP: TWILIGHT ZONE
  • 3,300 FEET DEEP: MIDNIGHT ZONE
    • 7,380 feet: Deepest recorded dive by a sperm whale
    • 12,450 feet: Depth of the Titanic wreckage
  • 13,100 FEET DEEP: ABYSSAL ZONE
  • 19,700 FEET DEEP: HADAL ZONE
    • 35,872 feet: Depth of Challenger Deep

1. Based on the text and pictures, what is a submersible? What do you know about the submersible O’Brien and Vescovo used?

2. What does the article say is an ocean explorer’s biggest enemy? Why?

3. What similarities does O’Brien note between Mount Everest and Challenger Deep?

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