The Deep Ocean

The ocean covers more than 70% of the Earth's surface, but most people only ever see the very top of it. Below the sunlit surface where fish swim and waves break, there is another world — a world of total darkness, crushing pressure, and freezing cold temperatures. This is the deep ocean, and it is the largest living space on our entire planet.

Scientists divide the ocean into different zones based on depth. The top 200 metres is called the sunlight zone — this is where most ocean life that we know about lives, because sunlight can reach it and plants can grow. Below that, from 200 to 1,000 metres, is the twilight zone — a dim, gloomy place where only a tiny amount of light can reach. Below 1,000 metres, there is no light at all. This is the midnight zone.

The deepest part of the ocean is the Mariana Trench in the Pacific Ocean. At its deepest point, called Challenger Deep, the ocean floor is nearly 11,000 metres below the surface. That is deeper than Mount Everest is tall — if you put Mount Everest at the bottom of the Mariana Trench, its peak would still be more than 2,000 metres under water.

More people have walked on the Moon than have visited the deepest part of the ocean. We have better maps of Mars than we do of our own ocean floor. Scientists estimate that more than 80% of the ocean has never been explored, mapped, or even observed by humans. In many ways, the deep ocean is the last great unexplored frontier on Earth.

The animals that live in the deep ocean are some of the strangest creatures on Earth. Because there is no sunlight in the deep ocean, they have evolved in ways that seem almost alien — with enormous eyes, transparent bodies, expandable stomachs, and the ability to create their own light.

One of the most famous deep-sea abilities is bioluminescence — the power to produce light from your own body. More than 90% of deep-sea creatures can glow in the dark. They use this light for many purposes — to attract prey, to scare predators, to communicate with each other, and to find mates in the total darkness. The anglerfish, for example, has a glowing lure that dangles in front of its enormous mouth, attracting smaller fish that swim towards the light and straight into its jaws.

The giant squid is one of the ocean's most mysterious creatures. It can grow up to 13 metres long — about the length of a bus — and has the largest eyes of any animal on Earth, each one the size of a dinner plate. For centuries, sailors told stories of enormous sea monsters attacking ships, and many scientists now believe these legends were inspired by encounters with giant squid.

Perhaps the strangest deep-sea creatures are the ones that live near hydrothermal vents — cracks in the ocean floor where super-heated water, rich in chemicals, shoots up from inside the Earth. The water near these vents can reach temperatures of 400 degrees Celsius. Despite these extreme conditions, entire ecosystems thrive around them — including giant tube worms up to 2 metres tall, eyeless white crabs, and bacteria that feed on chemicals instead of sunlight. These creatures have completely rewritten our understanding of where life can exist.

Exploring the deep ocean is one of the greatest challenges in science. The pressure at the bottom of the ocean is enormous — more than 1,000 times the pressure at the surface. At these depths, the water would crush a human body instantly. Even the strongest submarines need specially designed hulls to withstand the pressure.

The first people to reach the bottom of the Mariana Trench were Jacques Piccard and Don Walsh, who descended in a special vessel called the Trieste in 1960. They reached a depth of 10,916 metres and spent just 20 minutes on the ocean floor before returning to the surface. It took nearly 60 years before anyone went back — in 2019, explorer Victor Vescovo made the journey in a modern submersible called the DSV Limiting Factor.

Today, most deep-sea exploration is done by robots. Remotely operated vehicles, or ROVs, can dive to the deepest parts of the ocean, filming everything they see and collecting samples of water, mud, and creatures. These robots have made extraordinary discoveries — new species of animals, underwater volcanoes, ancient shipwrecks, and even traces of human pollution in the most remote places on Earth.

One of the most disturbing discoveries in recent years has been plastic. In 2019, a plastic bag was found at the bottom of the Mariana Trench — nearly 11 kilometres below the surface. Microplastics — tiny fragments of broken-down plastic — have been found in deep-sea creatures living in the most remote parts of the ocean. Even the most unreachable places on Earth have not escaped the impact of human activity.

The deep ocean might seem far away and unimportant to our daily lives, but it plays a crucial role in keeping our planet healthy. Understanding and protecting it is one of the most important challenges facing humanity.

The ocean absorbs about 30% of the carbon dioxide that humans produce by burning fossil fuels. Without the ocean acting as a giant carbon sink, climate change would be far worse than it already is. The deep ocean also stores enormous amounts of heat — it acts like a battery, absorbing excess warmth from the atmosphere and slowing down global warming. Scientists are only just beginning to understand how important this process is, and what might happen if it changes.

The deep ocean is also home to an incredible amount of biodiversity that we are only just beginning to discover. Scientists estimate that millions of species living in the deep ocean have never been identified. Some of these creatures produce chemicals that could be used in medicine — deep-sea sponges, for example, have been found to contain substances that may help fight cancer and other diseases.

But the deep ocean faces growing threats. Deep-sea mining companies want to dig up minerals from the ocean floor, which would destroy habitats that took thousands of years to develop. Overfishing is pushing fishing fleets into deeper and deeper waters. And climate change is affecting deep-sea temperatures, currents, and chemistry in ways that scientists are still working to understand.

The deep ocean is not just an interesting place — it is essential to life on Earth. Protecting it requires international cooperation, better science, and a recognition that even the places we cannot see deserve our care.