Out of sight must never be out of mind

1

The deep sea covers more ocean than you may realise

The deep sea refers to ocean depths beyond 200 metres, stretching down to almost 11,000 metres (yes, you read that correctly!)

Together, these depths cover just over half of Earth’s surface, making the deep sea the largest living space on the planet.

Scientists divide the ocean into depth-defined zones:

• Sunlight zone (epipelagic: 0–100 metres)
• Twilight zone (mesopelagic: 200–1,000 metres)
• Midnight zone (bathypelagic: 1,000–4,000 metres)
• The Abyss (abyssopelagic: 4,000–6,000 metres)
• Trenches (hadalpelagic: 6,000–11,000 metres)

The abyssal zone is one of the largest habitats on Earth.

Incredibly, across all land and ocean surfaces, the average altitude of Earth sits at around 4,000 metres underwater – a reminder of just how vast our oceans really are.

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It’s home to extraordinary life we’ve barely discovered

From glowing jellies and dumbo octopuses to ancient corals and giant sea spiders, the deep ocean is full of extraordinary life found nowhere else on Earth.

Scientists estimate there are hundreds of thousands to up to 1.5 million species [1] still waiting to be discovered in the deep. And with each expedition, new and often unexpected creatures are being uncovered all the time.

What’s even more remarkable is how interconnected these ecosystems appear to be. A recent study found that deep-sea invertebrates have quietly migrated across entire ocean basins over millions of years. Brittle stars (Ophiuroidea) — ancient, spiny marine animals — have been recorded from shallow coastal waters to the deepest abyssal plains, and from the equator to the poles.

The more we learn about these ecosystems, the more we understand their importance to the ocean as a whole.

And the more we discover, the clearer it becomes: the deep sea is full of life that deserves to be understood, valued and protected.

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We’ve only seen a tiny fraction of it – and ignorance matters

Our understanding of deep-sea ecosystems lags behind shallower marine environments, largely because of the scale and difficulty of deep-sea research.

In fact, scientists estimate we’ve visually observed only 0.001 per cent [2] of it.

This is not due to a lack of interest, but because of real challenges:

• Scale: The deep-sea spans around 70 per cent of the planet, requiring many or long expeditions to study.
• Technical: At full ocean depth, pressure reaches around one tonne per square centimetre, and temperatures are near freezing.
• Access: Research depends on specialised open-ocean vessels, of which there are relatively few.
• Finance: Deep-sea science is costly and difficult to sustain at scale.
• Perception: The deep sea is often treated as “out of sight, out of mind”, with its value still underestimated or overlooked.

Yet distance does not equal irrelevance. The deep sea plays a critical role in the functioning of the entire ocean.

4

The deep sea is intricately connected to life in shallower waters

When we think about the deep sea, it’s easy to picture the seafloor in isolation. But deep-sea ecosystems are part of a continuous system that links the seabed, the midwater column, and the surface ocean.

The ocean’s midwaters (roughly 200–1,000 metres deep) contain an immense reservoir of life — including vast biomass of fish and plankton that form the foundation of ocean food webs.

These midwater ecosystems help sustain species higher up the food chain, from commercially harvested species like tuna to deep-diving marine mammals.

This matters for deep-sea mining, because the risks aren’t limited to the seabed. Mining activity can generate sediment plumes, noise, and dissolved pollutants that spread through the water column and travel well beyond the mining site. [3]

These disturbances may disrupt the delicate balance of midwater ecosystems — and in turn, the food webs that connect the deep ocean to life much closer to the surface. That’s precisely why the deep seas must not be disturbed, especially before strong, science-based environmental safeguards are in place.

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Pacific Island communities are on the front line

For many Pacific Island nations, the deep ocean is not distant or abstract. It is central to culture, identity, and livelihoods.

Large areas targeted for deep-sea mining lie in the Pacific, including regions adjacent to or within the waters of island nations whose economies, food security, and traditions are deeply tied to a healthy ocean.

The risks of deep-sea mining do not respect political boundaries. Impacts in international waters may affect migratory species such as tuna, as well as marine ecosystems closer to shore, with real economic consequences for communities that rely on fisheries and tourism [4].

For many Pacific leaders, this is not just an environmental issue, but a question of fairness and sovereignty: who bears the risks, and who benefits from extraction? This perspective has helped drive growing calls from Pacific nations and civil society for a precautionary pause on deep-sea mining.

6

The deep sea plays a significant role in regulating our climate

The ocean is one of Earth’s greatest climate regulators.

It absorbs more than a quarter of human-caused CO₂ emissions and around 90 per cent of the excess heat generated by global warming.

It also holds an enormous store of carbon — an estimated 38,000 billion tonnes — far more than is stored in the atmosphere and on land combined.

Most of this carbon is stored at deep ocean levels, making the deep seas the planet’s largest and most secure carbon sink.

The deep ocean stores carbon via two distinct routes:

1. The solubility pump: Cold and dense waters absorb CO₂ at high latitudes and sinks, carrying that carbon into the deep ocean.
2. The biological pump: Phytoplankton (microscopic algae and other microorganisms) convert CO₂ into organic matter at the surface of the ocean. Some of that carbon sinks through the ocean’s midwater layers toward the deep sea — where it can remain for hundreds to thousands of years.

This is why what happens in the deep ocean matters so much. When deep-sea ecosystems are disrupted, the impacts don’t stay out of sight — they can ripple through entire ecosystems.

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Deep sea mining risks irreversible harm before safeguards exists

Time and time again, we’ve seen what happens when human activity moves faster than science and governance: ecosystems are damaged before we’ve fully understood what they sustain – and recovery can take generations.

The deep sea is one of the last places on Earth where that story doesn’t have to repeat.