Incredible as it may seem, we know little about 99 per cent of the inhabitable space on Earth. For - measured by volume, rather than just surface area - that is what the seas and oceans represent. And we know virtually nothing about their depths.

UPPER: A midwater hatchet fish Argyropelecus aculeatus, whose silvery flattened body is an effective camouflaging mechanism in the deep. Photo: Shaun Collin LOWER: The midwater dragonfish, Malacosteus niger, whose light organs emit red light most other deep-sea inhabitants cannot see, giving it a 'private wavelength'. Photo: Tammy Frank	UPPER: The lizardfish, Bathysaurus ferox, which lives on the ocean floor at depths of up to 4,000 metres, caught while eating. Photo: Ron Douglas LOWER: The deep-water pearleye, Scopelarchus analis, whose yellow eyes form upward pointing 'telescopes' to make best use of the dim remaining sunlight. Photo: Justin Marshall

Using nets is one possible solution, but getting one down 4,000 metres requires a line up to 14 kilometres long. It takes up to 12 hours to lower and raise and is difficult to control, and so risks damaging species samples. And the net is only about as big as a football goal, tiny compared to the immensity of the oceans.

Submersible craft provide an alternative, but there are only about a dozen suitable for the deep sea throughout the world, and sending one down even 2,000 metres is extremely expensive. Indeed people have only once reached the deepest part of the sea, when Jacques Piccard and Don Walsh plunged to the bottom of the Mariana Trench in 1960: the walls of their craft, Trieste, were 127 millimetres thick to withstand the huge pressure of 1.4 tonnes per square centimetre.

'Submersibles are very noisy and have bright lights in a quiet, dark place, and this scares specimens away,' says Douglas. 'Anything that has any sense will get out of the way, leaving scientists with only the stupid, blind and old to look at!'

Yet, despite all these difficulties, researchers are continually finding new life in the deep sea. 'The animals down there are endlessly fascinating,' says Douglas. 'They have adapted to the incredible pressure, as well as the extremes in temperature and the absence of light: few survive when they are brought to the surface.'

The density of creatures in the deep sea is low, so species have adapted to that too. Most fish have sharp teeth, big mouths and stretchy stomachs to increase their chances of catching and digesting any prey that ventures by. And some types of male anglerfish have developed a unique way of ensuring that they take any chance to reproduce: they attach themselves to a female's back - permanently.

In that dark world, Douglas explains, creatures communicate through light produced by specialized organs on the body called photophores. These are also used to lure mates or prey - and to scare away predators.

But this is just a glimpse. As Douglas points out, it is difficult enough to grasp an idea of all the creatures down there - let alone determine each one's function within its ecosystem. He is sure that the life of the deep ocean is at least as diverse as on the land - probably very much more so - and is just as likely to provide valuable products, such as life-saving medicines.

The bid to put a man on the moon began at about the same time as Piccard and Walsh reached the Mariana Trench. Since then, 12 humans have walked on it: none has been back to the sea floor. Douglas concludes, 'The ocean frontier will likely be as exciting to future generations as space travel was to those before.'

The world's seas, on average, are fully 4,000 metres deep (the deepest spot, as far as we know, is the Pacific's Mariana Trench, some 11,000 metres down). Everything beneath 200 metres is classified as the deep sea, and remains largely a mystery. So far we have explored just 10 square kilometres of the planet's 300 million square kilometres of ocean floor.

The little we have found is intriguing. For a start the seabed, like land, has plains and trenches, mountain ranges, volcanoes and canyons. Temperatures run to extremes: while most of the deep sea is icy cold, a few places are boiling hot. At these hydrothermal vents, scorching, poisonous water spews out of cracks in the sea floor. But despite the heat and toxic sulphides, many creatures - including giant tubeworms, clams and microorganisms - live around them.

The main obstacle to learning more about the deep sea is the difficulty of getting there, explains Ron Douglas of the University of Cambridge and City University London. Humans can barely dive down a mere 30 to 40 metres without specialized assistance. Pressure increases 1 atmosphere for every 10 metres. And it is pitch dark: sunlight only penetrates 1,000 metres down.