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How deep is the ocean?

Illustration of the marine sea floor with underwater plants, corals and fish.

We’re exploring a part of the world that not much is known about—in fact, you could be one of the people who help us understand and learn more about this very important, and very large, part of our earth.

The land underneath the ocean is as varied and interesting as the terrain up on dry land—with mountains and canyons, plains and forests. (That’s right, forests! There are kelp forests where the kelp is as much as 150 feet tall!) In this episode, what’s known--and unknown--about the bottom of the ocean. How deep IS the deepest part of the ocean? And how was the Mariana Trench formed? We get answers from Jamie McMichael-Phillips and Vicki Ferrini of Seabed 2030, a global collaboration designed to map the sea floor, by 2030.

Download our learning guides: PDF | Google Slide | Transcript

“How deep is the deepest part of the ocean?”
-Freya, 8, Wellington, New Zealand

The deepest part of the ocean is the Challenger Deep, 11,034 meters in the Mariana Trench. It’s about seven miles deep! How did the trench get so deep?

The same processes that formed canyons and mountains on dry land also formed the depths of the ocean and the islands that peek above the water.

In the case of the Mariana Trench, it was formed by the process of subduction—when one tectonic plate slides under another. A tectonic plate is a gigantic piece of the earth’s crust and the next layer below that, called the upper mantle. These massive slabs of rock are constantly moving, but usually very slowly, so a lot of changes to the earth’s structure take place over a long time. But sometimes something like an earthquake can speed that process up.

A trench is formed when one plate slides or melts beneath another one.

The Mariana Trench is the deepest trench in the world—farther below sea level than Mount Everest, is tall!

Jane Lindholm is the host, executive producer and creator of But Why: A Podcast For Curious Kids. In addition to her work on our international kids show, she produces special projects for Vermont Public. Until March 2021, she was host and editor of the award-winning Vermont Public program Vermont Edition.
Melody is the Contributing Editor for But Why: A Podcast For Curious Kids and the co-author of two But Why books with Jane Lindholm.
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