How do we know?
↻ One of the pages with a moving picture
Ships measure the depth by timing an echo.
- The top 200 meters (about 650 feet) is the sunlight zone. There’s enough light for tiny ocean plants to grow.
- From 200 to 1,000 meters down is the dim twilight zone.
- Below 1,000 meters it is completely dark. This is the midnight zone, and below that, the abyss.
- Deep cracks in the seafloor, called trenches, plunge below 6,000 meters.
Ships use sonar. They send a sound pulse down and time how long its echo takes to bounce back off the seafloor. A longer wait means deeper water.
Even so, only a little over a quarter of the seafloor has been mapped in detail.
The deepest places are in trenches. Challenger Deep in the Mariana Trench is named for HMS Challenger, whose crew first measured it in 1875. In 2022, a snailfish was filmed 8,336 m down in the Izu-Ogasawara Trench, south of Japan: the deepest fish ever seen!
Wow number
35,876 feet deep is Challenger Deep, the deepest known spot in the ocean. If Mount Everest sat at the bottom, its top would still be over a mile underwater!
Myth or fact?
The whole seafloor has been mapped in detail. Myth. Only a little over a quarter has been.
The ocean is over 12,000 feet deep on average. Fact. About 12,080 feet: over 2 miles!
Think about it
How do you think animals find food in the dark midnight zone?
Where in the world?
Challenger Deep, Mariana Trench, Pacific Ocean the deepest known spot in the ocean, named for HMS Challenger, whose crew first measured it in 1875. In 2022, a snailfish was filmed 8,336 m down in the Izu-Ogasawara Trench, south of Japan — 11.3733° N, 142.5917° E.
Where these facts come from
Every number on page 45, and who measured it
This page tells us how far sunlight reaches and names the ocean's zones.
How far does light travel in the ocean? — NOAA's Ocean Service, U.S. ocean scientists
oceanservice.noaa.gov/facts/light_travel.html
We checked it on 31 October 2026
This page tells us how deep the ocean is on average and how deep Challenger Deep is.
How deep is the ocean? — NOAA's Ocean Service, U.S. ocean scientists
oceanservice.noaa.gov/facts/oceandepth.html
We checked it on 31 October 2026
This page tells us how tall Mount Everest is.
Mount Everest is more than two feet taller, China and Nepal announce — National Geographic, a science and exploring magazine
nationalgeographic.com/science/article/new-height-of-mount-everest-announced-by-china-and-nepal
We checked it on 31 October 2026
This page tells us about the deepest fish ever filmed.
Snailfish sink to new lows to extend their record as the world’s deepest fish — Guinness World Records, which keeps lists of world records
guinnessworldrecords.com/news/2023/4/snailfish-sink-to-new-lows-to-extend-their-record-as-the-worlds-deepest-fish-743278
We checked it on 31 October 2026
This page tells us about the 2022 trip that filmed the deepest fish.
Scientists break new record after finding world's deepest fish — The University of Western Australia
uwa.edu.au/news/article/2023/april/scientists-break-new-record-after-finding-worlds-deepest-fish
We checked it on 31 October 2026
This page explains how sonar uses echoes.
What is sonar? — NOAA's Ocean Service, U.S. ocean scientists
oceanservice.noaa.gov/facts/sonar.html
We checked it on 31 October 2026
This page tells us how much of the seafloor has been mapped.
How much of the ocean has been explored? — NOAA's Ocean Service, U.S. ocean scientists
oceanservice.noaa.gov/facts/exploration.html
We checked it on 31 October 2026
Has this changed?
The ocean’s depth is not going to change, but what we know about it does. Only a little over a quarter of the seafloor has been mapped in detail.
Records change too: the deepest fish was filmed in 2022, and a deeper one could still be found.
Words from this page
- sunlight zone
- The top part of the ocean, where there is enough light for tiny ocean plants to grow.
- trench
- A very deep crack in the seafloor.
- sonar
- A way to measure depth with sound: send a pulse down and time its echo.
Try it!
Watch light fade in water.
1. Fill a tall, clear jar with water. Stir in a few drops of milk. 2. In a dark room, shine a flashlight down into the top of the jar. 3. Look at the jar from the side.
What happens: the light fades as it goes down. Sunlight fades in the sea the same way, until it is totally dark.
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