The twilight zone lies deep below the ocean surface, and the fish that live there could feed the world's population many times over – if we can work out how to catch them

Dive deep enough into the world's oceans and you will enter the twilight zone. This region, between 200 and 1,000m beneath the ocean's surface, earns its name because only the faintest hints of sunlight can reach such depths. The rest is absorbed and lost in the waters above.

Temperatures here can drop as low as 4C and water pressures reach about 10,000 kilopascals (1450 pounds per square inch), the equivalent of one person trying to support the weight of five Boeing 747s. With visibility levels almost zero, there is not enough light to support photosynthesis and so plants cannot exist at these depths.

Only creatures which have adapted to living in near darkness can survive. But despite the odds, life does not just persist in the twilight zone – it thrives.

A 2009 estimate suggests that the twilight zone is home to at least one million undescribed species, making it potentially one of the most biodiverse areas on the planet.

Some of these species are familiar: whales, for instance, often traverse the twilight zone in search of food. Sperm whales dive more than 4,000m down to catch giant squid which they locate using a form of biosonar that can pinpoint potential prey from thousands of metres away.

The ongoing cat-and-mouse game between predators and prey, played out in virtual darkness, has resulted in a world of the strange and bizarre

To survive in this most alien of environments, the creatures of the twilight zone have evolved in ways we can barely imagine.

Whales can withstand the immense pressure shifts between the surface and deeper layers of the ocean, changes which would easily break our fragile skeleton, because their lungs can deflate and their ribs are connected by hyper-flexible cartilage which allows the ribcage to safely collapse as they dive. Many of the fish, squid and eels which inhabit these depths have evolved to cope without the gas-filled swim bladders found in species living nearer the surface.

"Pressure mainly impacts on air bubbles in the body, it squeezes these down," says Michael St John, an oceanographer at the Technical University of Denmark in Charlottenlund who studies the twilight zone. "Think of snorkelling, your ears hurt as you dive down. If you don't have these spaces in your body, pressure doesn't really bother you all that much."

But it is not simply water pressure which has driven unique physiological adaptations in the twilight zone. The ongoing cat-and-mouse game between predators and prey, played out in virtual darkness, has resulted in a world of the strange and bizarre. And the deeper you go, the stranger it gets.

Many species have developed forms of "bioluminescence" – essentially ways to create their own light.

Humankind has sent far more explorers into space than to the deepest and darkest watery depths of our planet

Among the most common inhabitants of the deep ocean are lanternfish and the bristlemouths; a family of fish smaller than a finger with an immense gaping mouth and an array of needle-like fangs. Lanternfish have a set of organs which enable them to glow in the dark, while bristlemouths use a strategy known as counter-illumination to try and deceive their many predators.

In the dusky half-light during the day, it is difficult for predators to see downwards, but many track their prey from below by looking for their silhouettes. To protect against this sort of attack, bristlemouths have an array of bioluminescent spots on their bodies which can alter their colour and brightness to blend in precisely with any surrounding light, making their shadows disappear.

While this adaptation is somewhat effective, the bristlemouths' predators have responded in turn by evolving complex sensory systems to home in on their prey by detecting movement and vibration in the surrounding water.

But for the most part, the life cycles of these mysterious species remain unknown. Humankind has sent far more explorers into space than to the deepest and darkest watery depths of our planet, and for billions of years the twilight zone has been left largely untouched.

But that could be about to change.

Until relatively recently, some believed that domestic chickens were the most abundant vertebrates on the planet with numbers estimated at around 24 billion. In fact, this figure is dwarfed by some fish in the twilight zone. The global bristlemouth population is so vast, for instance, that numbers may lie in the quadrillions while various estimates of lanternfish suggest that their biomass alone is several times greater than the entire world fisheries catch.

There's enough fish there to make up 1.3 tonnes per human on the planet

Such is the abundance of these fish that they greatly perplexed oceanographers trying to measure the depth of the world's oceans using sonar during World War Two. The sonar signals reflected back off the fish, giving the impression of a "false ocean bottom", only a few hundred metres down. Once they realised what was going on, the US military considered trying to use these layers of fish as a camouflage to hide their submarines.

In total, based on new acoustic surveys, scientists now believe that the biomass of fish in the twilight zone is at least 10 billion metric tonnes.

"It's an immense number," St John says. "There's enough fish there to make up 1.3 tonnes per human on the planet. If you take and harvest 50% of that, turn it into fishmeal and then feed it to chickens through agriculture or pigs, you're creating 4.3 kilos of animal protein per human per day. So you've got people starving on the planet right now through food shortages, and here's a huge larder which we haven't even touched."

For years, catching these fish on a commercial scale has been dismissed as unfeasible both technologically and economically. But the steady realisation of the immense untapped resources at stake is leading more and more countries to take a greater interest in the potential cash cow lurking in the murky depths.

The majority of species which live in the twilight zone are not thought to be suitable for direct human consumption, simply because they are not particularly appetising. Instead the idea is to exploit them indirectly by using them as feed in agriculture production, the demand for which is growing by the year.

In 1995 the price of fishmeal was $500 (£404) per metric tonne, while by 2016, the steady depletion of existing resources and simultaneous population growth has seen this rise to over $2000 (£1615) a tonne. With global demand for animal protein predicted to increase by 30% between 2010 and 2030, and to continue growing exponentially thereafter, the need for fishmeal will only become increasingly more pressing.

So far it's not a major economic activity anywhere, due to the technical difficulties of it all

South Africa has long had an eye on exploiting the vast lanternfish community living in the twilight zone off the African continental shelf, to try and relieve the pressure on dwindling conventional fish stocks. Government reports suggest that half the species currently caught in South African waters are heavily overfished with some populations rapidly declining. In contrast, lanternfish biomass off the South African coast has been estimated at 18 million tonnes.

However one of the country's very first attempts, in the mid-1980s, to build an experimental fishery offshore in the Atlantic yielded an unexpected problem.

Lanternfish have an extremely high oil content, making them very hard to handle. Once the catch was aboard, it spoiled quickly in the high tropical temperatures, and began to degrade dangerously quickly. The temperature rose to such a high level in the decomposing fish that they spontaneously caught fire. An uncontrollable blaze swept through and destroyed the entire fishery plant.

In the subsequent 30 years there have been attempts to harvest the twilight zone off the coasts of India, Norway, Iceland, Japan and in the Arabian Sea. But these activities have been limited to an extremely small scale.

"So far it's not a major economic activity anywhere, due to the technical difficulties of it all," says Mike Heath, a population biologist at the University of Strathclyde in Glasgow, UK.

Aside from the problems in handling these fish, simply catching them is difficult using conventional techniques. While many species in the twilight zone move from deep water to the ocean surface at night, following the daily migration of zooplankton, they are extremely fast and tend to avoid nets.

There are regions of the ocean that are particularly nutrient-rich and so contain far greater populations in a concentrated area

"To catch them you've got to be prepared to filter a lot of water so you need very large nets and a large, powerful vessel to pull them," St John says. "And that costs money."

Catching enough to recoup shipping costs has also proved difficult as while fish in the twilight zone may be abundant, they are mostly sparsely dispersed through much of the world's oceans. Various assessments of twilight zone fish in the north-east Atlantic concluded that the fish were spread so thinly, the amount of fuel that would have to be burnt to catch an economically-viable amount of fish was too great for the time being.

Some scientists have been looking at ways to attract the fish so they aggregate in larger groups. Nature does something similar already: there are regions of the ocean that are particularly nutrient-rich and so contain far greater populations in a concentrated area. These "hot spots" are known as upwellings, and in recent years increasingly sophisticated imaging technology is helping us identify them more accurately than ever before.

Upwellings are zones on the maritime boundaries where ocean currents combine to drive nutrients from the depths to the surface layer, fuelling the production of phytoplankton. This abundant food source draws vast quantities of deep ocean fish.

So far, upwellings have been detected in various places across the world from Peru to Namibia. One particular hot spot is in the Sea of Oman where estimates have placed the twilight zone fish population at 20 million.

If harvesting these fish for nutriceuticals turns out to be economically viable, the ultimate consequences for the ocean ecosystem may be drastic

Oman has been looking to harvest the resource for many years. Their plans have been thwarted recently by the drop in oil price which has hit the local economy, but many think it is simply a matter of time before Oman takes the plunge – especially with the emergence of a new potential market for the fish.

The oil in the bodies of fish in the twilight zone is rich in "nutriceuticals" – foods, including fatty acids such as Omega 3, that are of benefit to human health. Much of the Omega 3 we consume comes from algae in the marine environment because terrestrial organisms cannot create the fat.

"At the moment when you look at fishmeal, right now it's a business where companies just try to break even or make a little bit of money," St John says. "But as soon as you look at the nutriceuticals, it changes the economic game completely."

However, if harvesting these fish for nutriceuticals turns out to be economically viable, the ultimate consequences for the ocean ecosystem may be drastic.

The blue whiting offers a cautionary tale for what may one day happen to lanternfish and other species. It was once one of the largest fish stocks on the planet, found in the twilight zone of the North Atlantic. But unlike lanternfish which spend much of their time in the murky depths, the blue whiting was a relatively shallow species which aggregated along the continental shelf in high numbers, and so was easier to catch. As a result it was decimated, largely to supply the cat food market in the UK.

Nobody's looked into the ecological impact of how fishing these fish will affect the entire food web

Blue whiting has been a key source of food for a whole variety of creatures in the ocean ecosystem ranging from tuna and seabirds to dolphins, sharks and whales. By depleting the stocks of such fish, we can indirectly hurt ourselves in ways which are not immediately obvious.

"Whales and dolphins are big in the ecotourism industry," St John says. "But they move in areas where there's very dense populations of blue whiting, and if you wipe their food out, that will change their migration patterns and so the ecotourism business suffers. We're seeing evidence of this now."

The twilight zone is particularly rich in biodiversity compared to the other layers of the ocean, and scientists fear we may be taking a risk by harvesting its species without fully understanding the consequences of their value to the overall health of the ocean and the predators that rely on them.

"Whales and large tunas migrate on ocean scales to feed in these upwellings where the plankton and twilight zone fish gather," Heath says. "Nobody's looked into the ecological impact of how fishing these fish will affect the entire food web."

Some believe that if done correctly, fishing the twilight zone could have a positive effect, especially on the overall health of the coastal regions where stocks of shallow water fish are particularly low. Fishing the deeper regions could relieve the pressure on the shallow water stocks currently used for fishmeal, and bring a more natural balance back to the coastal ecosystem.

"Before we dive into this we need more information on the reproductive rates of these twilight zone fish," St John says. "How many eggs do they produce? How many predators do they have? How much food is available to the adults? It's a very complex picture."

All that carbon which would have been in the surface layer has now been moved to depth

Studying the biology of the fish could help identify areas where the twilight zone fish are particularly healthy, with high growth rates, spawning rates and survival rates. Such areas would be more suitable for fisheries, says St John.

However, it is not just the ecosystem that concerns scientists.

Fish in the twilight zone have been found to be enormously important in terms of climate regulation. Their daily vertical migration from hundreds of metres down to the surface and back is part of a natural carbon pump, which sucks carbon out of the atmosphere and buries it deep in the ocean.

"These fish move up close to the surface layer to feed on zooplankton, which are in turn feeding on the phytoplankton up there," St John says. "Carbon dioxide gets into the ocean through surface mixing, and the phytoplankton take this up and use it to create sugars. And so it passes into the zooplankton and then these fish."

The twilight zone fish then return to deeper waters, where their food is digested and ultimately excreted. "So all that carbon which would have been in the surface layer has now been moved to depth."

Such is the amount of carbon the fish transport, scientists fear that removing them without proper understanding of the consequences could have a drastic effect on our climate.

"If that carbon dioxide stays in the surface layer, it gets transported back into the atmosphere which exacerbates global warming and increases global temperature," St John says. "This is not a trivial issue. And additionally, if we allow this to happen and the ocean temperature starts to increase, the ocean can't hold as much carbon dioxide. Warmer water cannot hold as much gas as colder water. And the problem spirals."

Everyone will be [thinking] 'I'll fish it hard because if I don't, somebody else is going to do it'

Some countries are already aware of the risks. The National Oceanic and Atmospheric Administration (NOAA) in the US has placed an embargo on any exploitation of twilight zone fish in American waters in the Pacific until more information has been gathered about the ecological consequences. But for some nations the economic pull may be too strong.

"The US is probably leading the way on being cautious and applying precautions and principles," Heath says. "But for some countries that have large areas of the deep ocean in their territorial waters and they're struggling to feed their population, then they're interested in looking out into their economic zone and seeing what they can harvest.

"Japan is a big consumer of seafood, the market is very strong for seafood there. They're also very interested in the nutriceutical angle of these resources, so wherever you look there are different pressures and incentives."

The Atlantic redfish provides another sorry example for could happen to species in the twilight zone. Huge biomasses of the redfish were found in the North Atlantic, but while the stocks were large, the sustainability was low as the redfish does not reach maturity until between 20 and 30 years old. Such was the intensity of the fishing that before long, the entire stock had been wiped out.

"If you do this to the twilight zone fish, you get into the global warming issue and you've got a big problem," St John says. "But as soon as they get the technology right, there's the potential for a gold rush there. Everyone will be [thinking] 'I'll fish it hard because if I don't, somebody else is going to do it'."

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