The twilight zone happens to be one of the final frontiers yet to be fully explored and exploited by humanity. This fascinating, deep underwater world with its weird, wonderful and terrifying inhabitants that lurk there, is vital to our oceans and ecosystems. Yet we know so little about it.

Read on to discover 30 incredible facts about our ocean's mysterious twilight zone...

The twilight zone spans the entire globe across all the oceans and around every continent and island. It covers millions of square miles and along with the rainforests, this underwater section of our amazing planet contains more animals and different species than anywhere else on Earth.

The twilight zone begins at 656 feet (200m) below the surface, around the point at which sunlight runs out. It is also known as the mesopelagic zone, meaning 'middle sea', and plummets to depths of 3,281 feet (1,000m). Below this you have three more layers – the midnight zone, the abyss and the trenches.

The temperature as far down as 3,000 feet (914m) can be as low as 5°C (41°F) and because of the incredible pressure and pitch-black darkness, the middle ocean is one of the hardest places to explore. Not to mention the inhabitants; not the type of enemies you want to encounter when it's more or less always a dark night.

Most animals in the twilight zone could be straight out of a science fiction horror movie. Or, your worst nightmares. In a world of big teeth, venomous spikes and gaping big eyes these weird and wonderful creatures have evolved and adapted to negotiate life in this unbelievably harsh and murky environment. Despite these inhospitable conditions, however, the twilight zone is teeming with some of the most diverse life on Earth, such as these translucent crustaceans, pictured.

Many animals in the twilight zone and below are capable of producing their own light – a process called bioluminescence. As the name lanternfish suggests, this enables these small fish to camouflage themselves in order to avoid predators, and also to look for their own dinner in the dark. Lanternfish, of which there are over 245 species, are also endowed with extra photophores on their side. This suggests an extra superpower – to communicate with each other. 

The ocean contains more than 50 times more carbon than the atmosphere. Animals from the twilight zone who 'commute' to the surface at night and return to the depths during the daytime play a vital role in transporting and storing carbon dioxide from the atmosphere. This is due to the amount of carbon in the debris, algae and prey that is consumed. This is part of a process known as the 'biological carbon pump'.  

Phytoplankton play a pivotal role in the ocean's ecosystems and can be monitored effectively from space by satellite. This algae/bacteria provides food for virtually every animal, either directly or indirectly, as it recycles through the marine food chain. The biological carbon pump, (with phytoplankton providing food at the surface of the ocean for many twilight zone creatures), is therefore an important area of study for scientists to understand the world's carbon levels. 

If you've ever read about creatures of the deep, the aptly named fangtooth, with its protruding lower fangs, is probably the one fish you have seen plenty of times. Opportunistic hunters with a ferocious appearance, these fish are equipped with special slots in their upper jaws so as not to skewer their own brains when chewing. 

Water pressure in the twilight zone can reach 1,500 pounds (680kg) per square inch, with 0.6 miles (1km) of water weighing down above its furthest depth. This intense pressure is equivalent to 10 elephants standing on your head.  

One of the major activities that threatens not only the twilight zone but the oceans in general is carbon dioxide removal – taking carbon dioxide out of the atmosphere and storing it on the seabed. Whilst the ocean does this naturally with the process known as the 'biological carbon pump', there is more research being done on how much of a negative impact human intervention will have.

Another threat to the twilight zone is deep sea mining – drilling the seabed for nodules. These are golf-ball-sized spheres rich in the metals needed for technology, such as electronics. The debris and waste from mining these nodules will almost certainly have a negative impact on the ecosystems.

Despite being surface predators on the whole, predatory fish such as great white sharks, whale sharks, tuna and swordfish are thought to often dive deep into the twilight zone and even further into the midnight zone at 3,280 to 9,843 feet (1,000 to 3,000m). However, this is not just to search for prey, and scientists are still unclear as to why they dive much further than the levels at which they feed. 

As there is barely any sunlight within this layer of the ocean, plants cannot grow due to the need for photosynthesis. Some animals that live here have little to feed on and tend to have an almost sluggish lifestyle due to the poor energy sources available. These smaller creatures survive on the debris, leftovers and bacteria that float down from creatures in the upper layer of the ocean, the sunlight zone.

The surface water of the planet's oceans contains around one billion tonnes of fish; however in the twilight zone it is thought that the number could be 20 times that – more than the rest of the entire ocean combined. This inevitably means that in the future, fishing the twilight zone as a potential food source could be a possibility.

The strawberry squid, another bizarre and uniquely evolved beast, graces the twilight zone with its presence but has a special feature. This particular cephalopod is cock-eyed, with a blue eye almost half the size of its yellow one. This helps it to make out the different spectrums of light as it propels itself through the ocean looking for prey. Another characteristic is the squid's posture, which enables simultaneous viewing.  

Although 71% of the Earth is covered by water, 95% of this is still unexplored. The twilight zone, and its fascinating bounty of life remains, on the whole, still to be researched properly. As do the depths further down, the midnight zone and beyond. Experts say that we know more about the Moon than we do the deep oceans.

A fish that is as close to a living fossil as you will get. Once thought to be extinct, the coelacanth is one of the twilight zone's oldest members, seeming to have avoided evolution for the past 400 million years. This primitive creature is nevertheless critically endangered, with one of the two remaining species presumed to have dwindled to less than 500 in numbers.

These are the extinct animals that once roamed our planet

During the 1940s, US Navy sonar reports caused a lot of confusion. Noticing a change in depth from the ocean floor, operators presumed that the seabed was shifting. It turned out to be the daily migration of animals, plankton and so on that was causing the mass movement on the radars.

Such is the vastness of the unknown depths, it is estimated that the deep oceans, much like the rainforests, could see animal species become extinct before they have even been discovered. This is due to the sheer magnitude of exploration involved. Another obvious issue is the expense. Even an unmanned submersible is thought to cost up to £7,575 ($10,000) per day. 

Scientists have recently discovered that ocean currents known as 'eddies' regularly develop in the planet's oceans. These are the marine equivalent to atmospheric storms. The eddies cause warm water, which provides top predators in the surface regions – sharks, whales and tuna – with a pathway to the twilight zone, which might otherwise be too cold for them to access. 

Some animals leave the twilight zone on a daily basis to feed on prey, plankton and other microorganisms near the sunlight surface. In doing so, it helps them avoid daytime predators. This phenomenon, called 'the Diel Vertical Migration', is the largest migration of animals on the planet. Billions of sea creatures endure this dark and dangerous daily commute. Yet we barely know that it even happens...

The sperm whale is one of the ocean's biggest dwellers, and one of the more familiar ones. Its battles with its adversary the giant squid are not just the stuff of folklore, and the dark murky waters of the twilight zone provide the most atmospheric of backdrops to this age long rivalry. Sperm whales are a frequent visitor to the twilight zone, indeed to prey upon giant squid, and the fact that this is one of the deepest diving mammals on the planet means it can delve even further into the abyss.

A lot of the strange residents of the twilight zone are minute, and only measure a few inches long. However others, such as the gelatinous siphonophores (pictured) are some of the longest animals in the world, and can grow more than 130 feet (40m) in length. Although these ultra long specimens are venomous, they are also very fragile and can break into many pieces upon the slightest contact. 

It is extremely difficult to research the twilight zone as unlike the surface ocean, which can be imaged remotely and is accessible by ships, the twilight zone is simply too vast. Scientists believe that what we know about the twilight zone right now, is as much as what we do not know.
Also due to the darkness, special lighting is necessary for filming, not to mention specialist vessels to withstand the immense pressure. 

Most of the world's gelatinous creatures, like the moon jellyfish, call the twilight zone home. Unlike many other jellyfish, this particular species does not contain poison. It is also capable of reducing its size to almost 10% in order to save energy – a perfect adaptation to life in the tough world of the mesopelagic.

The amount of debris from dead creatures and waste from living ones on the surface of our oceans creates 'marine snow'. This snow sinks in the ocean and takes a large amount of carbon as it falls down. Some animals within the twilight zone feast on this debris, meaning that 90% is recycled within. The remaining 10% of marine snow sinks down even deeper, helping to maintain the levels of carbon dioxide.  

There are thought to be over 1,000 trillion (or even quadrillion) bristlemouth fish in the ocean, which would make it the most abundant vertebrate on Earth. That's 100,000 for every human. Astonishingly, there are also many species yet to be discovered. At lengths smaller than your finger, bristlemouth can also glow in the dark, which is useful for hunting prey and fending off predators.

This fearsome apex predator is the dragonfish. Aesthetically speaking, one could argue that this monster of the deep is as petrifying as it gets. An actual dragon might look less intimidating than this alien-like specimen. There are a number of different types, with varying characteristics. One particular shared trait is their ability to consume prey much larger than their own size.

A strong spine (and a stiff upper lip) is needed to survive down here, and the slender snipe eel has more bones than any other animal on Earth – over 750 vertebrae on average. Whilst living in most of the ocean's zones, it spends most of its time in the mesopelagic.

Whilst most marine life in the twilight zone is considered otherworldly and straight out of the horrors of one's imagination, there are more familiar names that you will likely have heard of. Equally as terrifying as any image you can concoct is the giant squid, for example. With eyes that are the biggest in the animal kingdom measuring around 10 inches in diameter, these enable this gigantic mollusk to see in the murky depths. These animals can even grow longer than a bus. 

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