The ocean facts on this page were each checked against a named source: a page published by the U.S. National Oceanic and Atmospheric Administration (NOAA), or a study in a peer-reviewed scientific journal. Every card links to that source and shows the date the fact was verified. Where the source gives an estimate, the fact says it is an estimate.
That last point matters more for the ocean than for almost any other subject. A great deal of what is said about the sea is measured, and a great deal is estimated, and a good fact tells you which is which.
One ocean, and the shape of it
We talk about “the oceans”, and maps give them separate names. NOAA’s own answer to how many there are is shorter than you might expect.
There Is Only One Global Ocean
There is only one global ocean, which covers 71 percent of the Earth and is divided into named regions.
Of its named regions, one is far larger than the rest.
The Pacific Holds More Than Half of Earth's Free Water
The Pacific Ocean contains more than half of the free water on Earth, and all of the world's continents could fit into its basin.
The floor of the ocean is not flat. It holds the deepest place on the planet, and NOAA gives its depth in both meters and feet.
The Ocean's Deepest Known Point Is Nearly 11 km Down
The deepest known part of the ocean, the Challenger Deep in the Mariana Trench, is approximately 10,935 meters (35,876 feet) deep.
It also holds the longest mountain range on Earth, most of which nobody will ever see from land.
Earth's Longest Mountain Range Is Mostly Under the Ocean
The longest mountain range on Earth is the mid-ocean ridge, which spans 40,389 miles around the globe, with about 90 percent of it under the ocean.
Water that behaves strangely
Seawater is not just water, and the salt changes how it behaves.
Seawater Is About 3.5 Percent Salt
About 3.5 percent of the weight of seawater comes from dissolved salts, which come mainly from rocks on land and openings in the seafloor.
Because of that salt, the sea has to get colder than a lake before it freezes.
Seawater Freezes at About 28.4 Degrees Fahrenheit
Seawater freezes at about 28.4 degrees Fahrenheit, lower than the 32 degrees at which fresh water freezes, because of the salt in it.
Salt is not the only thing dissolved in it. There is gold in seawater too, although NOAA is clear that nobody can extract it at a profit.
There Is Gold Dissolved in the Ocean
Ocean water contains gold, but it is so dilute that one study found only about one gram for every 100 million metric tons of water in the Atlantic and north Pacific.
Cold water is denser than warm water, and that simple fact produces something that sounds impossible: a waterfall inside the sea.
The World's Largest Waterfall Is Under the Ocean
The world's largest waterfall lies beneath the Denmark Strait, between Iceland and Greenland, where cold water plunges nearly two miles.
The ocean also has rivers of a kind. One of them carries more water than every river on land put together.
The Gulf Stream Moves More Water Than All the World's Rivers
The Gulf Stream transports an amount of water greater than that carried by all of the world's rivers combined.
Tides are not the same everywhere, and the record belongs to one bay in Canada.
The Highest Tides in the World Are in Canada's Bay of Fundy
The highest tides in the world are found in Canada's Bay of Fundy, at Burntcoat Head in Nova Scotia.
And a wave set off by an earthquake can cross deep water at a speed that is hard to picture.
Tsunami Waves Can Travel as Fast as a Jet Plane
Over deep water, tsunami waves may travel as fast as jet planes, slowing down only when they reach shallow water.
Life in the sea
The ocean does more for life on land than most people realise. NOAA reports that scientists estimate about half of the oxygen produced on Earth comes from it. Note the word “estimate”: it is in the source, so it is in the fact.
About Half of Earth's Oxygen Comes From the Ocean
Scientists estimate that roughly half of the oxygen production on Earth comes from the ocean, mostly from tiny photosynthetic plankton.
The largest structure built by living things is in the sea, and it is not made by people.
The Great Barrier Reef Is Earth's Largest Living Structure
The Great Barrier Reef is the largest living structure on Earth, stretching for 1,429 miles over an area of approximately 133,000 square miles.
The reef is built by corals, which are easy to mistake for plants or rocks.
Corals Are Animals, Not Plants
Corals are animals, not plants: most coral structures are made up of hundreds to thousands of tiny creatures called polyps.
Several familiar sea creatures are not what their names suggest, or lack things we assume every animal has.
Starfish Are Not Fish and Have No Blood
Sea stars, commonly called starfish, are not fish: they have no gills, scales or fins, and they use sea water instead of blood to pump nutrients through their bodies.
Jellyfish Have No Brain, Blood or Heart
Jellyfish have no brains, blood or hearts, and their bodies are about 95 percent water.
And in one family of fish, it is the father who carries the young.
Male Seahorses Get Pregnant and Give Birth
Seahorses and their close relatives, sea dragons, are the only species in which the male gets pregnant and gives birth.
Two facts from peer-reviewed studies
Not every ocean fact comes from an agency’s public pages. Some come straight from research. A peer-reviewed study is one that other scientists in the field examined before the journal published it. That makes it a primary source, and we rank it Tier A. It does not make it the last word, so we report what the study found in the study’s own terms.
The first is about age. In 2016 a team used radiocarbon dating on the eye lenses of Greenland sharks and published the results in the journal Science. The key words in the fact are “at least”.
The Greenland Shark Is the Longest-Lived Vertebrate Known
The Greenland shark is the longest-lived vertebrate known: radiocarbon dating of 28 sharks revealed a life span of at least 272 years.
The second is about how much is still unknown. A 2011 study in PLOS Biology estimated the number of species on Earth and in the ocean from the way known species are classified. It is a prediction, with a stated margin of error, and our fact presents it as one.
About 91 Percent of Ocean Species Have Not Been Described
A 2011 study estimated that about 2.2 million species live in the ocean and that some 91 percent of them still await description.
How these facts were checked
For the NOAA facts, an editor opened the NOAA page, found the sentence that supports the claim, and wrote the claim so that it says no more than that sentence. For the two studies, the claim was compared with the published abstract. In each case the source, its tier and the date are recorded and shown on the fact’s page, with a confidence score out of 100.
Checking also means noticing when sources disagree. While preparing these facts we found two NOAA pages that give different figures for the area of the Pacific Ocean. Neither figure appears in our Pacific fact, which keeps to what both pages say.
FactFusion AI uses AI assistance for content organization, explanation drafting, and classification. All facts are verified by human editors before publishing.
The full process, and what each tier means, is on our page How we verify facts. You can browse every ocean fact in the Ocean category, or let the random fact generator choose one for you.
Questions and answers
How many oceans are there?
One. NOAA says there is only one global ocean, divided into named regions. Most countries, including the United States, now recognize five: the Atlantic, Pacific, Indian, Arctic and Southern.
What is the deepest point in the ocean?
The Challenger Deep, in the Mariana Trench. NOAA gives its depth as approximately 10,935 meters, or 35,876 feet.
Is there really a waterfall under the ocean?
Yes. NOAA says the world’s largest waterfall lies beneath the Denmark Strait, between Iceland and Greenland, where cold, dense water sinks below warmer water and flows over a drop in the ocean floor.
What is the longest-living animal with a backbone?
The Greenland shark, according to a 2016 study in the journal Science, which found a life span of at least 272 years.
What does “peer-reviewed” mean?
It means other scientists in the same field examined the study before the journal published it. It is a check on the method, not a guarantee that the result will never be revised.