12 Weird Ocean Facts: Underwater Rivers, Glowing Animals and More

The ocean covers about 71% of Earth's surface and holds around 97% of its water — and we know surprisingly little about most of it. Here are twelve facts that show how strange it is down there.

1. There are rivers on the seafloor

Dense, salty or sediment-heavy water can flow along the bottom of the ocean like a river, beneath lighter water above. These underwater channels carve canyons, build banks and even form deltas. One off the Black Sea coast is estimated to carry more water than many major rivers on land.

2. Most of the ocean is unexplored

According to NOAA, more than 80% of the ocean remains unmapped, unobserved and unexplored in detail. We have better maps of the surface of Mars than of much of our own seafloor.

3. The longest mountain range is underwater

The mid-ocean ridge system winds around the globe for about 65,000 km (40,000 miles) — far longer than the Andes or the Himalayas. It's where tectonic plates pull apart and new seafloor forms.

4. The deepest point is deeper than Everest is tall

The Challenger Deep in the Mariana Trench reaches roughly 10,900 m. Drop Mount Everest (8,849 m) in and its peak would still be more than 2 km underwater. The pressure there is over 1,000 times that at sea level.

5. Many deep-sea animals make their own light

Around three-quarters of animals in the deep open ocean are thought to produce light. Anglerfish dangle glowing lures; some squid squirt glowing ink; lanternfish use light patterns to recognise each other. This bioluminescence is the most common form of communication on the planet.

6. There's gold in seawater

Seawater contains dissolved gold — spread across the oceans, it adds up to millions of tonnes. But it's so dilute (roughly a few grams per hundred million tonnes of water) that extracting it costs far more than the gold is worth.

7. Coral reefs are built by animals

Corals look like plants or rocks, but they're colonies of tiny animals called polyps, related to jellyfish and sea anemones. Reefs cover less than 1% of the seafloor yet support around a quarter of all known marine species.

8. The ocean produces much of the oxygen you breathe

Tiny marine plants and bacteria — phytoplankton — produce roughly half of the oxygen in the atmosphere through photosynthesis. One of them, Prochlorococcus, may be the most abundant photosynthetic organism on Earth.

9. Sound travels a very long way underwater

Sound moves about four times faster in water than in air. A layer called the SOFAR channel traps sound so effectively that low-frequency noises can be detected thousands of kilometres away — whales may use it to communicate across ocean basins.

10. There are lakes and waterfalls under the sea

Where extra-salty brine seeps from the seafloor, it pools into brine lakes with shorelines and even waves. And the Denmark Strait cataract, where cold, dense water plunges over an undersea ridge, is the largest known waterfall on Earth — about 3.5 km tall.

11. The Pacific is bigger than all land combined

The Pacific Ocean covers about 165 million km² — more than all of Earth's land area put together.

12. The ocean is older than the dinosaurs — by a lot

Evidence from ancient minerals suggests liquid water existed on Earth's surface over 4 billion years ago, not long after the planet formed.

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