Deep Sea

The deep seas are one of the least explored environments on Earth and have been visited by fewer people than have been to the moon. The deep sea includes all water below 200m which is an area of over 300 million km2, more than 30x larger than the area of the USA.

The deep ocean covers approximately 60% of the entire Earth’s surface, making it the most common ecosystem on Earth, but it remains one of the least understood and known of all ecosystems. It was once believed to be a barren lifeless environment where nothing could possibly exist. It is now well-known that this environment supports a diverse and very interesting set of animals.


The absence of light is a significant factor that influences the environment of the deep oceans. There is a tiny amount of light between 200-1000 m in what is sometimes referred to as the twilight zone. Beneath 1000m light is virtually absent and the environment is in a state of perpetual darkness.

Below 200m the level of light is too low to support phytoplankton and other photosynthetic organisms. The only visible light is that produced by living things through bioluminescence. Around 70% of all animals within the deep oceans use bioluminescence for a range of purposes. Many fish will use it to attract prey and mates, other animals will also use bioluminescence to help prevent being preyed upon by using it to disguise their silhouette or as a distraction tool when they are being pursued by a predator.


The temperature in the deep oceans is relatively stable in comparison to many other environments. Typically the deep oceans range in temperature between 1-6°C and do not experience any seasonal changes. The exceptions are areas of geothermal activity where water is heated by the heat that has escaped from beneath the Earth’s surface. These locations are known as hydrothermal vents and cold seeps.

At hydrothermal vents, the water can be super-heated to above 400°C due to the extreme amount of pressure on the water preventing it from boiling. Hydrothermal vents are relatively short-lived but provide a large amount of thermal energy to the area over a short period. Cold seeps are far more stable environments where the geothermal activity is less vigorous. They last for a lot longer and provide more stable environments.

These two environments are the only known locations to support ecosystems where the primary energy is not sourced from the sun. Bacteria at these locations are able to use the thermal energy to convert gases into carbohydrates and provide the foundations of amazingly diverse and abundant ecosystems surrounding the geothermal hotspots.


The pressure deep within the ocean is immense and can pose serious problems for life and exploration through the deep oceans. At the water’s surface, the pressure is the standard atmospheric pressure e.g. 1 atm. For every 10m of depth, the pressure increases by an additional 1 atm. So at 200m, the pressure is around 21x greater than it is at the surface and at 1000m the pressure is approximately 101 atm or 101x times greater than the atmospheric pressure that we are accustomed to.

Water itself is not affected too much by increases in pressure, but gas-filled spaces can be crushed if pressure increases too much. This creates significant problems for life deep within the oceans, for diving animals that feed at depth and for developing technology that can explore the world’s deep oceans. Diving animals, for example, have to deal with gases compressing within their body as they travel into deeper water. To overcome this issue, animals such as the sperm whale have evolved the ability to safely collapse their lungs and reduced air spaces within sinuses during dives.


Due to the absence of light, there are no living plants or phytoplankton within the deep oceans. Instead, food webs are generally driven by surface debris that sinks to the bottom of the ocean. This is a very sporadic food source but many animal species still manage to survive. Invertebrates dominate the sea floor, in particular, echinoderms such as starfish and sea cucumbers. Jellyfish, sponges, polychaetes, and crustaceans are also common.

Hydrothermal vents are dominated by tube worms, clams, and crabs. There is a vast array of interesting fish species (such as anglerfish, viperfish, gulper eel, and blobfish) that look very different to what we are used to in shallow waters. There are also many species that look relatively similar to the fish we are more used to such as lanternfish, cookiecutter shark, orange roughy and Antarctic toothfish.

How deep does the ocean get?

The Mariana Trench, located in the western side of the Pacific Ocean (not far from the Philippines and Japan), has the deepest spot in the ocean. Within the Mariana Trench, a location known as the Challenger Deep sits beneath 11km of ocean water. At that depth, the water pressure is around 1100 times greater than the atmospheric pressure.

Last edited: 28 November 2018

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