How Deep Is the Ocean?

mariana trench — The depth of the ocean on planet Earth is not uniform. It varies a great deal on the basis of geography. DOERS/Shutterstock

To know the ocean is to know some mind-boggling numbers.

About 71 percent of Earth's surface is covered by seawater. All that liquid takes up more than 332,519,000 cubic miles — or 1,386,000,000 cubic kilometers — of space in total.

The official website of the U.S. National Oceanic and Atmospheric Administration says there's enough water in the ocean to "fill about 352,670,000,000,000,000,000 gallon-sized milk containers!"

Now let's go out on a limb and assume you don't have that many spare milk jugs lying around. Here's another crazy visual: In theory, you could use the ocean to cover the entirety of the United States (yes, that includes Alaska and Hawaii) in a column of water measuring over 82 miles (i.e., 132 kilometers) tall.

Pardon us for using a four-letter word, but ... dang!

It's no surprise that some of the most frequently asked questions about our ocean involve its depth. How deep is the ocean? Where's the very deepest part of it? And what's the ocean's average depth?

You'll learn the answers to those inquiries about the deep sea today, and as a bonus, you won't even need to pack your scuba gear.

The Average Depth of the Ocean

Strictly speaking, Earth's only got one ocean. So it's a little confusing that the five major regions within this giant body of water are all called "oceans" in their own right: there's the Pacific Ocean, the Atlantic Ocean, the Indian Ocean, the Arctic Ocean and the Southern Ocean.

Together, they make up what's known as the "global ocean." That's what people usually mean when they start talking about "the ocean" as a whole. (It's also what we were referring to in both of the hypothetical scenarios described earlier.)

Of course, the depth of the ocean is not uniform. It varies a great deal on the basis of geography. At any random spot on the map, the distance between the ocean floor and the water's surface up above may be influenced by canyons, undersea mountain ranges or a host of other features.

Using such tools as sonar, radar and satellite technology, scientists have calculated that the global ocean has an average (mean) depth of approximately 12,785 feet (3,897 meters). That works out to around 2.4 miles, or 3.8 kilometers.

The Deepest Parts of the Ocean

Mariana Trench, Everest — The Mariana Trench is even deeper than Mount Everest is tall. The deepest spot is called "Challenger Deep" after the HMS Challenger, a British Navy ship that found the Mariana Trench in 1875. The Challenger crew collected samples that laid the foundation for deep ocean research. VectorMine/Shutterstock

We can also compare different sites. A 2019 study published in the journal Earth-Science Reviews used the available data and research to break down the deepest zones in each of the global ocean's five subregions. Here's what the authors found:

The deepest point in the Arctic Ocean is a place called Molloy Hole. It's located 18,599 feet (5,669 meters) below the surface.
The deepest part of the Indian Ocean? According to the study's authors, that's probably an unnamed part of the Java Trench situated 23,917 feet (7,290 meters) underwater.
As for the Southern Ocean, which encircles Antarctica, its deepest locality can be found within the South Sandwich Trench, at a depth of 24,229 feet (7,385 meters).
With a depth of 27,585 feet (8,408 meters), a Puerto Rico Trench site known as Milwaukee Deep is the deepest part of the mighty Atlantic Ocean.

"But wait!" you say. "What about the Pacific Ocean?" Fear not, we've saved the best for last. East of the Mariana Islands, a Pacific archipelago, there's a yawning underwater chasm that adventurers and science fiction writers can't get enough of.

Its name is the Mariana Trench and it contains the very deepest place in not only the Pacific, but in the entire global ocean as well, a site that's labeled on our maps as "Challenger Deep." The trench is located in the western Pacific Ocean.

Measuring its exact depth has proven tricky, but by a rather conservative estimate the 2019 study endorses, the sea floor here rests an incredible 35,843 feet (10,925 meters) below the water's surface.

How deep is Challenger Deep? Plenty deep. Indeed, this Pacific point is even deeper than Mount Everest is tall. The peak of that Himalayan landmark is only around 29,026 feet (8,848 meters) above sea level. It's the movements and interactions of Earth's tectonic plates that form these extreme trenches, as well as the tallest mountains.

Now That's Interesting

Jules Verne's "Twenty Thousand Leagues Under the Sea" is a sci-fi masterpiece. Yet the 19th-century text has a misleading title. A "league," as Jules Verne knew it, is equal to 3 nautical miles, the equivalent of 1.1508 "land" miles (1.863 kilometers). So, 20,000 leagues is about 60,000 (landlubber) miles or 116,000 kilometers. As we've seen, no part of the global ocean approaches that depth in real life. But Verne wasn't talking about depth here. His evocative title references a fictional journey that covered 20,000 leagues horizontally.

Frequently Asked Questions

What technologies are used to measure the depths of oceans like the Mariana Trench?

Modern technologies for measuring ocean depths include sonar mapping, which uses sound waves to detect the sea floor's contours, and deep-sea submersibles equipped with pressure-resistant depth gauges. Satellite altimetry, which measures the surface bulges of water over deep areas, also provides indirect depth measurements.

How does deep ocean exploration impact our understanding of Earth's biology?

Deep ocean exploration reveals unique ecosystems, such as hydrothermal vent communities, that thrive in extreme conditions without sunlight. These discoveries expand our knowledge of biodiversity, biological resilience and the possibilities of life in extreme environments, informing both science and conservation efforts.