Ironically, outer space is better explored than three-quarters of our own planet. Brian Bett looks at the murky secrets of the deep

Recently I published a children's science book. Its title - Planet Ocean - reflects the fact that nearly three-quarters of our planet is under water. The deep seas can tell us much about problems, like climate change, that afflict the earth, yet the underwater world, at its deepest points, remains remarkably underexplored. This is so despite the fact that marine biology can guarantee to find strange new life forms for a fraction of the cost of exploring outer space.

One recent finding is that the deep-sea floor has its own seasonal cycle, a discovery that could be vital in the attempt to understand global warming. New information is emerging, too, about unfamiliar animals, whose bodies have adapted to crushing water pressure. Most of the creatures on the ocean floor are unknown. In the late 1970s a massive deep-water shark called the Megamouth was spotted for the first time when it got tangled up with oceanographic equipment. Although it was several metres long, it had gone unnoticed because of the depth at which it lived.

I based Planet Ocean on an imaginary tour of the deep-sea floor by submersible - a mini-submarine strong enough to withstand deep-water pressure. Promoting the book, I was asked the same questions several times. Do you dive? "Yes, but for fun, not for work. Scuba gear will take you 50 metres into the ocean but my work is at 5,000 metres." Have you been down in a submarine? "Yes, but only to 300 metres." Why don't you use a submersible then, and how do you manage to work at 5,000 metres without one?

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At this point I usually reach for two facts. First, that the United Kingdom does not have a deep-sea submersible (they are extremely costly). Only the Russians, Americans, French and Japanese have them. Second, and paradoxically in an age of advancing technology, nobody has a submersible that can reach the greatest depths of the ocean (11,000 metres), though the feat was achieved 40 years ago by two scientists, Jaques Piccard and Don Walsh.

So, without a submersible, how do we British marine scientists compete with our counterparts, who work for the oceanographic superpowers? The answer is that we have become adept at using other forms of technology. Current projects include an unmanned free-swimming vehicle - an autosub - and landers, as in Mars landers, dropped from ships into the ocean to observe the daily activity of the deep-sea floor.

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One of the most popular subjects of submersible-based science are hydrothermal vents, rare spouts of super-heated water found on mid-ocean ridges, that give off chemicals such as methane gas. These can be "sniffed out" by chemical sniffers towed deep in the water behind ships and then monitored by cameras lowered on the end of wires or by acoustic systems that produce detailed maps of the seafloor's topography and texture.

Nevertheless, the fact remains that, to carry out detailed investigation of the exotic animal communities that live around these chemically fuelled oases or to map out specific details of hydrothermal vent floors, we have no technology better than sending skilled biologists and geologists down in a submersible.

Why are vents a key focus for so much research? Well, over most of the ocean floor animals are rather thin on the ground, but around vents they thrive. This is because of the availability of food in different parts of the ocean. Away from vents, animals rely on food produced by photosynthetic organisms - plants, the algae that make up the phytoplankton of the ocean's surface, which can convert the sun's energy into living matter. Around vents, chemosynthetic bacteria produce food without the need for light, turning simple chemicals such as hydrogen sulphide from the vent water into more complicated substances that animals can feed on. Some of these bacteria live free on the rocky terrain of the vents where they are grazed by larger creatures such as shrimps, others live within the body tissues of highly specialised vent animals, such as giant tube worms and clams.

Chemosynthetic communities also develop around "cold seeps", where fluids containing methane and hydrogen sulphide leak from the seafloor. But vents and seeps are short-lived homes. Their stock of chemosynthetic fuel inevitably dries up. Perhaps the most novel deep-sea chemosynthetic communities are those found around whale bones. The fall of a whale carcass to the ocean floor provides a short-lived bonanza for scavenging fish and shrimps that rapidly reduce it to a clean skeleton. Later, a chemosynthetic community may develop, fuelled by organic molecules that slowly leach out from inside the bones.

But 99.99 per cent of the ocean floor is neither a hydrothermal vent nor a cold seep, though you would be forgiven for thinking otherwise judging by the attention they get. It is a visual problem; given the choice between an exotic vent scene and an expanse of barren, flat mud, most of the ocean floor usually does not make the cut. Yet the ocean's muddy bottom is the world's largest environment.

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Where does this mud come from? Most of the deep sea lies beyond the reach of the muddy waters spewed from the world's rivers. The mud of the ocean floor is a several hundred metre-thick pile of skeletons - of chalk, of glass, exquisitely sculptured. These remains, of the mostly microscopic plankton that once inhabited the ocean's surface, accumulated at an achingly slow rate over millennia and hold a record of the ocean's past climate. But not all of the mud arrives in this gentle, endless rain, some is dumped on the ocean floor by cataclysmic undersea landslides.

Whether delivered as a gentle rain or a catastrophic slide, it is this mud that fuels life on the deep-sea floor. In this realm of permanent darkness, the animals depend on the plants miles above to make the food they need. As on land, these plants may have particular growing seasons. In the oceanic waters around the UK, plants put on a burst of growth in spring that produces an unexpected bonus for the animals below. Although there are many hungry mouths between the sea surface and the seafloor miles below, and although, in the weeks it takes for the dying plants to fall to the bottom much decomposition takes place, an unusually large package of food arrives on the seafloor in late spring/early summer.

This deep-sea seasonality is a relatively recent discovery. Although it is possible that some of this decomposing plant material was collected off Ireland during Britain's first deep-sea expedition in the 1870s, it was then though to be primordial slime, the stuff that fostered the origins of life. Today the true nature of the slime can be revealed by leaving a time-lapse camera on the ocean floor for a year. Through the winter and spring the seafloor is its normal muddy self, but suddenly, in early summer, green gunk carpets the bottom in a matter of days. Through the summer and autumn, this compost-like material is wafted around the seafloor in tidal currents and is gradually consumed by the animals of the deep-sea floor.

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The fate of this plant debris is now the subject of much research. Plants at the surface draw down carbon dioxide from the atmosphere, some of which must end up buried in the ocean's muddy floor. Since increased levels of carbon dioxide in the earth's atmosphere intensify the greenhouse effect, scientists are understandably keen to find out how much carbon gets buried in the ocean floor's mud. What happens when there is more or less carbon dioxide in the earth's atmosphere? Some of the answers to these sorts of questions are already recorded in the mud. But reading the record is far from straightforward. The simple message in the rain of debris is translated into other languages by chemical changes at the seafloor and is scrambled by the pesky deep-sea critters that insist on disturbing the neat layering of the sediment.

The relationship between increased levels of carbon dioxide and climate change is of global concern; recent years have also seen some well-publicised local concerns for the deep-sea environment. The most publicised of these was the thwarted attempt to dispose of the Brent Spar, a defunct North Sea oil storage tank, by dumping it in deep-water to the west of Scotland. Today environmental protest groups have shifted their attention to big business's exploration for oil and gas in the Atlantic Frontier, the deep waters to the north and west of the UK. This debate has already got off on the wrong foot, with a great deal of fuss about the possible damage drilling for oil might cause to a coral called Lophelia pertusa, a deep-water animal found along the European Atlantic coast.

Coral has an exotic ring to it, but in fact there are numerous species of coral found around the UK. None of these species forms coral reefs, which are restricted to the shallow, warm, sunlit waters of the tropics. The deep-water coral, Lophelia, can and does, off the Norwegian coast, form large structures some tens of metres across and high. But recent newspaper reports describing the existence of a giant barrier reef off the northwest of Scotland that deserves protection from the drillers are false. Lophelia does not appear particularly significant to the ecology of the Atlantic Frontier.

The deep waters around the UK are clearly important to science and commerce - do we now need a submersible to research this challenging environment? Oceanographic scientists would love one but we are realistic and recognise that our wishes are seldom met within our budget. To have a submersible we would have to lose something else - probably one of our ocean-going research ships - of which there are only three. That would be too high a price to pay. Strange as it may seem, for the immediate future, outer space will remain better explored than our earth's own largest habitat - the deep-sea floor.

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Brian Bett is a deep-sea biologist based at the Southampton Oceanographic Centre.