The deep sea (defined as depths of over 200m) is a 'dark' secret; for centuries fishermen and those living on coasts have regaled stories of strange and wondrous beings ranging from mermaids and sirens to sea monsters. These have captured the world's imagination in countless ways, from films as disparate as "Jaws" and "The Little Mermaid" to classic novels such as "Moby Dick" and "2000 leagues under the sea".

But in recent years more sophisticated fishing methods have led to the legend of giant squid becoming a reality; any such unusual catches are widely reported in the press and the giant squid currently being exhibited at the Natural History Museum in London has a waiting time of several weeks for those wishing to purchase tickets to see it. So why is there such a great fascination for the deep ocean? The aquatic environment is the largest habitat on Earth; conversely it is also the one we know least about. Over 70% of the Earth's surface is covered with water; on land most organisms live within a few metres of the ground, whereas in the sea they can live in depths of up to 4000m. This means that over 99% of the Earth's habitable space is the deep sea - possibly the world's most diverse and important habitat. Why do we know so little about such a vast part of our planet? This article will look at the difficulties of studying the deep oceans, how these difficulties are being overcome and how far research has pursued the world that lies beneath the 200m mark.

The deep sea is a world very different to our own, pressures are several hundred times greater than those which are experienced on land, the temperature rarely rises above 4°C and it is pitch black as sunlight can only penetrate to depths of 1000m. Scientists can use nets to catch specimens of deep ocean life but to get a net down to 4000m requires a line 14000m long which can take 12 hours to lower and raise. It is difficult to control and risks samples being damaged, also it is only the size of a football goal - tiny compared to the size of the oceans. More rigorous research necessitates the use of highly sophisticated technologies, such as submersibles. Submersibles are small craft either manned or unmanned which can take samples and film the underwater world. There are only about a dozen submersibles throughout the world suitable for the deep sea and sending one down only 2000m is extremely expensive. Nevertheless they have proved very useful at discovering deep sea habitats. Both Professor Ron Douglas (City University) and Dr Jason Hall-Spencer (Plymouth University) use submersibles for their research into the visual ecology of fish and cold water coral respectively.

Hall-Spencer has examined fishing by-catches and used submersible video to look at cold water coral along western Ireland and western Norway. A great deal is known about shallow marine ecosystems but little is known about deep sea coral. Following the expansion of large scale fishing operations along the northeast Atlantic in the 1980s (due to the declination of shallow water fish stocks) it has been discovered that the deep sea fishing trawlers are causing significant damage to coral systems, including scars up to 4km long. This is causing concern as the corals are built up over centuries to millennia; the main north Atlantic species Lophelia pertusa grows between 2-25mm a year and slows down with age; so reef accumulation is slow. Although the existence of deep sea corals has been known for centuries, ecologists know very little about them; even basic knowledge such as how they feed or reproduce is very limited. Indeed, it is only in the last five years that video recordings taken by submersibles have revealed just how spectacular these reefs can be. The EC habitats directive applies to UK continental shelf waters up to a limit of 200 nautical miles, however, off-shore deep water reefs of all EU countries remain unprotected. Hall-Spencer's research focuses on the damage to the corals and what needs to be done to protect them, if they are destroyed before scientists are able to study them thoroughly the knowledge they can provide us with will be lost.

Similarly, little is known about the creatures that live in the deep sea. Douglas researches deep sea fish, this can be difficult as submersibles are very noisy and have bright lights in a quiet, dark place; often this can scare away the fish, leaving scientists with the ones which are blind, old or stupid. Despite these constraints Douglas has been able to study types of fish never seen before. He focuses on their adaptations to the deep sea environment, including the great levels of pressure, extremes in temperature and lack of light. His specialisation lies in the visual modifications of deep sea fish. The deep ocean is a blue place; both residual sunlight and bioluminescent emissions (the light given of by the fish themselves to search for prey and communicate) are on a wavelength of 450-500 nm. One species of fish however, Malacosteus niger (figure 2, left)), emits a red bioluminescence, as it cannot be detected by other species it is effectively a "private line" for communication and illumination of prey without the prey being aware of them.

The importance of the deep sea ecosystem is great; the life it contains is at least as diverse as that on land and likely to provide us with as many useful materials - such as food and medicines - as its terrestrial counterparts. The first mission to the deepest part of the ocean; the Mariana Trench was undertaken by Jacques Piccard and Don Walsh in 1960 - the same year plans were put in action to send a man to the moon. Since then 12 humans have walked on the moon but no one has returned to the deepest part of the sea; the secrets it holds are still waiting to be discovered...