Twilight

The deepest a human lung has gone

214 mHuman

Herbert Nitsch rode a weighted sled to 214 metres and came back on a lift bag. Four minutes and twenty-four seconds. At that depth his lungs were compressed to the size of a fist.

AIDA No-Limits record · 22.3 bar

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The seafloor that moves

300 mLifeEvery night

In 1942 sonar found a bottom that rose every night. It was not the bottom. It was billions of fish and squid climbing toward the surface under cover of darkness — the largest animal migration on Earth, and it happens daily.

Deep scattering layer · possibly 90% of the world's fish biomass

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↓ Most fish in the ocean live here

The limit for a human with equipment

332 mHuman

Ahmed Gabr reached 332.35 metres in the Red Sea. Going down took twelve minutes. Coming up took thirteen hours and thirty-five minutes.

The ascent is 65 times longer than the descent — that is the price the ocean charges

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It keeps fishing with no one holding it

411 mTrace

Gear that is lost or discarded goes on catching things as it sinks. The net does not break down, and it does not stop.

Abandoned, lost or discarded fishing gear (ALDFG) — estimates vary widely between bodies

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More ships made the sea louder

443 mSound

Engines and propellers overlap the low frequencies whales use. For an animal that speaks in sound, this is not fog. It is noise.

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Most fish in the ocean live here

475 mLife

The total mass of small fish in the twilight zone has been estimated to exceed all the fisheries above it combined. They are simply too small and too deep to have been caught yet.

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↑ The seafloor that moves · ↓ It erases its own shadow

Carbon, sinking

500 mOcean

Dead plankton and faecal pellets rain down from the sunlit layer without pause. That drift carries billions of tonnes of carbon into the deep every year and locks it there for centuries. Marine snow is not weather. It is the carbon cycle.

The biological pump · an estimated 5 – 12 Gt of carbon sequestered annually

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It erases its own shadow

600 mLife

Hatchetfish carry rows of light organs along their bellies. They match the faint glow filtering down from above, brightness for brightness, so that a predator looking up sees no silhouette at all.

Counter-illumination · the output is tuned as the ambient light changes

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↑ Most fish in the ocean live here · ↓ A colony 45 metres long

A colony 45 metres long

630 mLife

A siphonophore was filmed hanging in a spiral off Western Australia, the outer ring alone measuring 47 metres. It is not one animal but thousands of genetically identical clones fused into a single body with divided jobs. The longest animal on Earth.

Genus Apolemia · Ningaloo Canyons expedition

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↑ It erases its own shadow · ↓ The fish with a transparent head

The fish with a transparent head

720 mLife

The barreleye's skull is a clear fluid-filled dome. Inside it, two green tubular eyes stare upward, then rotate forward to track prey. The two dots you take for eyes are its nostrils.

Macropinna microstoma · first filmed alive in 2004

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↑ A colony 45 metres long · ↓ A shark that lives 400 years

Why water looks blue

760 mOcean

Water molecules absorb red light and let blue light bounce back. The ocean looks blue because of the water itself, not because it reflects the sky.

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A shark that lives 400 years

800 mLife

The Greenland shark outlives every other vertebrate. It takes 150 years to reach maturity, and some individuals swimming today were born while Newton was alive. Most are nearly blind, their eyes occupied by a parasite.

Radiocarbon-estimated lifespan 272 – 512 years

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↑ The fish with a transparent head

The layer where sound does not fade

920 mSound

At one depth, temperature and pressure combine so that sound travels slowest. Sound entering it cannot escape upward or downward, and carries for thousands of kilometres. It is the channel whales use to call across an ocean.

SOFAR channel — around 1,000 m at mid-latitudes

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Other markers

• 430 m Oxygen minimum zone — oxygen falls below 5% of surface levels here. And the zone is expanding. →
• 535 m Emperor penguin maximum dive — the deepest any bird has gone →
• 900 m About 76% of the animals at this depth make their own light — in the ocean, glowing is the default →

The limit of its era

• 923 m Bathysphere — the limit of human reach

What fits in this depth