Deep sea

To boldly go: In 1960, two men travelled five hours to the bottom of the sea for a 20-minute visit.

It’s in our DNA to discover: we’ve been around the globe – and even ventured into space. But there is one final frontier still to explore. It’s deep below the surface of the world’s oceans.

  • What is the deep sea?

    Far below the ocean’s surface is a world of darkness and cold. Deep sea officially begins at a depth of 200 metres, where fading sunlight creates a perpetual twilight zone. Lower down, the temperature drops and the pressure rises. A mile below sea level there is never any sunlight, water is below freezing – and the pressure would kill a human being in seconds.

  • How low does it go?

    The average depth of the entire ocean is 3.5km (2 miles), but underwater, the land is just as varied as it is on shore miles above. Mountains surge from underwater plains, volcanos throw molten magma into the icy waters, and streams of heavy brine slink along the seabed and pool into mysterious submarine lakes. Narrow, steep-sided canyons drop down far lower than the average depths. They are known as hadal trenches after the Greek god of the underworld.

    The deepest of these is the Mariana Trench. Located in the western Pacific between Japan and the Philippines, it is more than 1,500 miles long. At the southern end, a valley in its floor drops to nearly 11,000 metres (nearly 7 miles) below sea level. Known as the Challenger Deep, this point is so low that if Mount Everest were placed in the trench, it would still be more than a mile underwater.

  • What’s down there?

    Because – incredibly – there is life, scientists have been sending specially designed submarines deep into the ocean for more than 30 years. Almost every time, they discover a new species. Specially adapted species of jellyfish, sea worms, crabs, and even sharks thrive in the pitch-black midnight zone thousands of metres underwater.

    These strange creatures are almost alien-like in their appearance, with supple bodies to withstand pressure and huge eyes and mouths to spot and capture prey. Some, like the terrifying anglerfish, produce their own light with a chemical process called bioluminescence.

  • What does the deep sea teach us?

    In the freezing depths where sunlight never goes, hot springs erupt from fissures in the rock, spurting out gasses hundreds of degrees in temperature. It seems an unlikely environment for anything to live, and yet whole communities have grown up around the vents. Crustaceans, fish, and plants thrive on the surrounding rocks, requiring no light to survive, only the minerals created by the vents.

    Research shows these vents, located at the meeting points of tectonic plates, also release hydrocarbons. Scientists now believe the dark ocean floor could be where life on Earth began.

  • Is it just about discovering new life?

    No. The deep sea is also a rich source of minerals. For more than a century, oceanographers have identified minerals on the seafloor – copper, nickel, silver, platinum, gold, and some gemstones. The value of the minerals on the seabed is almost limitless. It is estimated that the gold alone could be worth $150 trillion – enough to buy whole countries and transform the global economy.

    As a result, global mining companies are developing technology for a new underwater gold rush. However, less than 10% of the ocean’s floors have been mapped, making it a place more alien to humans than the surface of the Moon. With so much of this underworld left to discover, mining in the deep ocean could have unknowable impact.

  • Does deep-sea exploration affect us?

    Yes! Unlocking the mysteries of deep-sea ecosystems reveals new sources for medication, food, and energy resources. The challenges of exploring the deep ocean have provided technology and engineering innovations that can be used in other situations. Information gathered can help predict earthquakes and tsunamis. Plus, it is helping scientists understand how we are affecting and being affected by changes in the Earth’s environment.

You Decide

  1. Is it a good idea to use the deep sea for mining?


  1. Hollywood science-fiction films often look to the deep for their alien beasts and landscapes. Using the resources in Become An Expert, do some more research into deep-sea creatures. Then design a monster inspired by the deep for a new sci-fi movie.

Word Watch

Never ending or changing.
Twilight zone
The lowest level of the ocean to which light can penetrate. Also means a situation or idea area that is undefined or mysterious.
A mile underwater, the pressure is about 100 times greater than it is at sea level, similar to the pressure on the surface of Venus.
A flat, sweeping landmass.
Molten magma
Molten (liquid) rock is called magma. When the magma cools enough, it solidifies and igneous rock forms.
Salty sea water.
The ocean is divided into zones based on depth. The lowest is the hadal zone, named after the Greek god of the underworld and his kingdom, both called Hades.
Bends and moves easily and gracefully; flexible.
This is a bony fish named for its characteristic mode of killing its prey: a fleshy growth dangles from its head, lit up to attract smaller creatures.
A chemical reaction that produces light energy within an organism’s body. For a reaction to occur, a species must contain luciferin, a molecule that produces light when it reacts with oxygen.
Long, narrow openings made by cracking or splitting, especially in rock or earth.
Spaces that allow air to pass through.
These are molecules critical to life. Being able to produce building blocks of life makes vents strong contenders as places where life might have originated on Earth.
Someone who studies the ocean.
Gold rush
A large-scale and quick movement of people to an area where gold has been discovered.
A series of waves in water caused by the movement of a large volume of water, generally in an ocean or a large lake. Earthquakes, volcanic eruptions, and other underwater explosions above or below water can all cause a tsunami.

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