1. Oceans make up 97% of all the water on the surface of the Earth.
It’s one of the most frequently-asked questions about the world’s oceans: how much of Earth’s water is in the oceans? The answer: almost all of it, with oceans making up 97% of all the water on the planet – that’s 320 million cubic miles. The world’s oceans also cover 71% of the Earth’s surface. But what about the other 2 most-asked questions…
What is the largest ocean? Though Earth really has just one large ocean, over time it’s been split up into five bodies of water: the Pacific Ocean (the largest and deepest), the Atlantic Ocean (second-largest), followed by the Indian Ocean, the Arctic Ocean and the Southern Ocean (or the Antarctic Ocean).
How deep is the ocean? The average depth of the ocean is approximately 3.7km (2.3 miles). However, the deepest point of the ocean (that we know of) is Challenger Deep in the Mariana Trench, at approximately 11km (for context, Mount Everest is 8.8km tall).
2. The longest mountain range on Earth is underwater
Spanning 40,389 miles around the globe, the Mid-Ocean Ridge is the longest mountain range in the world – and around 90% of it is under the ocean. The mountain chain plays a fundamental role in the process of plate tectonics, influencing the creation of new oceanic crust and the distribution of seismic activity.
3. World Ocean Day is held every year
World Ocean Day is held on June 8 each year to remind people across the world of the importance of the ocean, as well as inform the public of the impact of human actions on the ocean, develop a worldwide movement of citizens for the ocean, and unite the world’s population for the sustainable management of the world’s oceans. Each year has a theme – 2025’s theme is Sustainable Fishing Means More, highlighting the importance of sustainable fishing practices in ensuring the health and abundance of marine resources for generations to come.
4. The ocean is incredibly biodiverse
While we know the ocean to be home to over 240,000 species, it’s estimated that there are millions more as of yet undiscovered seeing as much of the deep sea hasn’t been explored yet (more on that later). Much of this biodiversity is microscopic bacteria and plankton, with just one liter of water from the ocean known to hold up to 38,000 kinds of microbes.
5. The ocean provides nearly half of the oxygen on Earth
Thanks to photosynthesizing plankton, the ocean produces nearly 50% of all oxygen on Earth – but that’s also roughly the same amount that’s consumed by marine life. However, the ocean also absorbs over a quarter of human-induced carbon emissions, having absorbed 90% of the warming that has occurred in recent decades due to increasing greenhouse gases. The top few meters of the ocean store as much heat as Earth’s entire atmosphere.
6. Coral reefs provide shelter for over 30% of marine species…
Coral reefs are crucial for marine ecosystems, sheltering close to a third of known marine species – in turn providing a direct source of livelihood for more than 500 million people worldwide through fishing. This is even more impressive when you consider that coral reefs are found in a tiny percentage of global oceans, between 0.08 and 0.16%.
8. …But 50% of coral reefs have been lost in the last 40 yearsocean-climate.org estimates that approximately 20% of the global coral reefs have already disappeared completely, while 25% are in high danger and 25% more will be threatened by 2050 if positive management action is not taken. The Great Barrier Reef, the world’s largest living structure, has been victim to mass coral bleaching events in 1998, 2002, 2016, 2017, 2020, 2022 and 2024 due to rising sea temperatures and ecosystem degradation. If the average global temperature rises by 1.5°C, the estimated loss of tropical coral reefs globally is 70% – 90%.
8. Oceans have their own natural barriersThe ocean has evolved its own defence system again shoreline erosion in the form of coral reefs, mangroves, seagrasses and saltmarshes. These natural coastline barriers buffer shorelines against 97% of the energy from waves, storms and floods, protecting not just the marine life and ecosystems that thrive on the shores, but human settlements on shore and further inland.
9. Sea water contains about 3.5% salt
Why is the ocean salty? There are two reasons. Firstly, because of openings in the sea floor like underwater volcanoes and hydrothermal vents, which directly release minerals into the ocean. Secondly, because of rain runoff; when slightly acidic rain falls, it erodes rocks and releases the mineral salts into rivers or streams that, ultimately, reach the ocean.
10. There is enough salt in the oceans to cover the whole surface of the Earth.
Think that 3.5% doesn’t sound like that much salt? Think again. If someone could remove all the salt in the ocean and spread it across the Earth’s land surface, it would form a layer more than 500 feet (166 meters) thick – that’s half the height of the Eiffel Tower. That has to be one of the best facts about the ocean for your next trivia night.