Time is running out to protect our alien and wonderful underwater world

The ocean is the largest ecosystem on Earth with a volume of 1.3 billion cubic kilometres of water and reaching nearly 11,000 metres in depth at its deepest point, the Challenger Deep in the Marianas Trench, in the western Pacific. This trench was first discovered by HMS Challenger in what many regard as the first modern oceanographic expedition, which set out from Sheerness in December 1872.

Since the Challenger expedition, we have learned much about the ocean, how it circulates, how it regulates the temperature, weather and atmospheric gases of the Earth and about the basic distribution of life within it. However, the further and deeper we go from the shore, the less we know. Only three people have visited the bottom of the Challenger Deep [including movie director James Cameron] compared to 12 who have visited the Moon. Much of the deep ocean we have never even seen, let alone studied, and we have mapped less than a fifth of the seabed.

Yet life thrives even at the greatest depths.

My own exploration of the ocean has taken me to the North Atlantic, the Indian Ocean and the Antarctic. I have dived to two miles below the surface of the Indian Ocean in a submersible that more resembled a space capsule than most people’s idea of a submarine. The absence of breathable air, extreme pressure and cold make the ocean a hostile place for human life, and the technological demands of exploring the deep make even the simplest scientific observations difficult. Yet life thrives even at the greatest depths.

The lights of the submersible on my dive revealed sheets of black basalt, cast into ropey piles as part of the process of creating new seabed. Everything was coated in a thin covering of bright white sediment, resembling pristine snow, and there to welcome me was a large red shrimp gently swimming up to investigate this alien visitor from above. It looked like another world, with no evidence that humans had ever been present.

Almost every time I have explored new places in the ocean the expedition has returned with many new species, everything from seaweeds to a new species of yeti crab that occurs in piles around hot springs in the deep Southern Ocean, known as hydrothermal vents. I have also seen evidence of the destruction humankind has brought to the ocean.

On underwater mountains in the Indian and Atlantic oceans, the cameras of remotely operated vehicles – robots we use to survey the sea floor – have revealed deep-sea coral reefs obliterated by bottom trawls. These have been deployed in the deep ocean hunting for species like the orange roughy, a fish that lives for 150 years and unsurprisingly has proved vulnerable to overfishing. Some deep-sea corals have been found to live for over 4,000 years and one sponge to 11,000 years old: they are slow growing and slow to reproduce, so the ability of such ecosystems to recover from fishing damage is very limited.

Even around the UK we have seen dramatic changes in our coastal seas. The English Channel and the North Sea are seeing an increase in warm-water fish species such as horse mackerel, anchovy and lesser weaver fish. Species we associate more with cooler waters such as cod, mackerel and sand eels appear to be moving north. Species never previously recorded in British waters have appeared more and more regularly over the last few decades, including exotics like the short-snouted seahorse. However, along with this increase in the diversity of our species, fish are becoming smaller, and fish stocks less productive. There have already been clashes between countries over who has the right to fish stocks which are moving north, most notably mackerel.

Whilst the news about the ocean has been extremely gloomy there are reasons for hope. Marine species can bounce back in spectacular fashion even after they have nearly been driven to extinction. I have seen some of these species myself, most notably the Antarctic fur seal, nearly exterminated for its dense pelts by the end of the 19th century but now thriving around the island of South Georgia in the South Atlantic where there are about four million individuals.

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The ocean also has not suffered from the same levels of extinction as land ecosystems. However, if we are to avoid a global-level extinction event in the ocean, there is no room for complacency. In the final chapter of The Deep I outline measures we need to take collectively as nations and we as individuals can take to turn the ocean around from decline to recovery; changing our diet; avoiding single-use plastics; trying to reduce our use of household goods that contain harmful chemicals and buying sustainably caught fish.

We can also teach ourselves about the ocean, what life it contains and why it is so important. Maybe consider joining an organisation that is working on ocean conservation. Ocean degradation is a big issue for all of us and everything we do can contribute to creating a healthy ocean and ultimately a healthy planet.