Underwater vehicle used for diving

The submersible used for the dive is named Limiting Factor.

© Triton Submarines; Shutterstock.com (Ocean Exploration 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

Vanessa O’Brien was headed into the unknown. It was June 2020, and O’Brien and pilot Victor Vescovo were crammed inside a small deep-sea vehicle called a submersible.

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

O’Brien and Vescovo’s sub 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. After four hours, Vescovo said, “Touchdown!” 

They had finally reached the deepest known spot 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

A TIGHT FIT! 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. O’Brien points out that we have more detailed maps of the surface of Mars than of our own oceans. 

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 is strong enough to easily crush a person and even most submarines. In the Mariana Trench, the pressure is like having the weight of 50 jumbo jets pressing down on your body.

To withstand the crushing pressure, Vescovo had his submersible built with thick walls made from one of the strongest metals on Earth. In 2019, Vescovo became just the fourth person to ever reach Challenger Deep. O’Brien was only the ninth. As a comparison, about 5,000 people have reached the top of Everest. 

Exploring the Seafloor

Jim McMahon/Mapman®

The Mariana Trench is enormous—it’s about five times as long as the Grand Canyon. During their expedition, O’Brien and Vescovo focused on gathering data to map a mile-long stretch of Challenger Deep, at the trench’s southern end. They also collected water samples and took photos and videos.

The intense pressure, total darkness, and near-freezing temperature make it difficult for most creatures to survive at that depth. But some life has adapted to the harsh environment. 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 Extremes

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

“As different as people would see these two places, they are similar in so many ways,” she says.

But O’Brien points out that you don’t need to journey to the world’s most extreme places to be an explorer.

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

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 the biggest enemy of ocean explorers? Why?

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

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