How deep is the ocean?

The remote-controlled Deep Discoverer vehicle captures images of a newly discovered hydrothermal vent field in the western Pacific. NOAA
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How deep is the ocean
Explorers began creating navigational maps that show how wide the ocean was more than 500 years ago. However, it is much more difficult to calculate how deep it is.
If you want to measure the depth of a pool or lake, you can tie a weight to a string, lower it to the bottom, then pull it up and measure the wet part of the string. In the ocean you would need a rope that is thousands of feet long.
In 1872 the HMS Challenger, a ship of the British Navy, set sail to learn more about the ocean, including its depth. It carried 291 kilometers of rope.
During their four-year voyage, the Challenger crew collected samples of rocks, mud and animals from many different areas of the ocean. They also found one of the deepest zones in the western Pacific, the Mariana Trench, which stretches for 2,540 kilometers.
Today scientists know that the ocean is on average 3.7 kilometers deep, but many parts are much shallower or deeper. To measure depth, they use sonar, which stands for Sound Navigation And Ranging. A ship sends out pulses of sound energy and measures depth based on how fast the sound travels back.
The deepest parts of the ocean are trenches - long, narrow depressions, like a trench in the ground, but much larger. HMS Challenger surveyed one of these zones at the southern end of the Mariana Trench, which is possibly the deepest point in the ocean. Known as the Challenger Deep, it is 35,768 to 36,037 feet deep - nearly seven miles.
Oceanologists like me study the ocean floor because it helps us understand how the earth works. For example, the outer layer of our planet is made up of tectonic plates - huge moving plates of rock and sediment. The Hawaiian-Emperor Seamount Range, a series of peaks on the ocean floor, was formed when a tectonic plate moved over a point where hot rock rose from the depths of the earth.
When two tectonic plates move away from each other underwater, new material rises into the earth's crust. This process of creating new seabed is known as seabed expansion. Sometimes super-hot fluids from the Earth's interior shoot through cracks in the ocean floor called hydrothermal vents.
Amazing fish, shellfish, tube worms, and other life forms live in these zones. Between the formation and destruction of sea plates, sediments accumulate on the sea floor and provide an archive of the earth's history, the evolution of the climate and life that is not available anywhere else.
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This article was republished by The Conversation, a non-profit news site dedicated to exchanging ideas from academic experts.
Continue reading:
Meet the sea creatures who use a slime tissue to catch their food
Scientists have been drilling the ocean floor for 50 years - here's what they found so far
The deepest living fish in the sea is small, pink and tender
Suzanne OConnell does not work for any company or organization that would benefit from this article, and does not consult or receive funding from stocks. She has not disclosed any relevant affiliations beyond her academic appointment.

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