Hadibo, the largest town on Socotra and home to several thousand people, was our base, with its spectacular backdrop of the Haggier mountains. But most of our time was spent travelling around the island standing in the back of an open pick-up truck, holding on for dear life as we made our way along boulder-strewn tracks, or wallowed through deep sand, often at little more than walking pace. Around our feet bounced compressor, tanks and all the other paraphernalia of a marine biological survey. Our two Socotrean drivers, Khamis and Mubarak, were at first a little perplexed by our antics whenever we stopped to look at a site. Our running in and out of the water and minutely examining the creatures and plants of the shore was not very obviously a useful way to pass the time. Once they got used to the idea they took it all in their stride, and Mubarak was clearly interested in the whole exercise and became an invaluable help. By the end of the two weeks he was something of an expert.

Everywhere we went we passed through small villages whose inhabitants were unfailingly friendly and welcoming. We ate fresh-caught fish in the evenings, either provided by the villagers or when possible, that we had bought earlier, at a previous stop along the coast. We were often conscious that we might be eating more in one evening than one of the villagers might eat in three, and so in spite of being ravenously hungry after a long day's work we tried to hold our appetites in check! We sometimes stayed in the village guest-house, customary on the island and literally a house or room set aside for the use of guests visiting the village. We more usually slept in the open, sometimes in the courtyards of small stone houses, and on one occasion in a small, open-air mosque consisting of little more than circular stone walls, with a carefully prepared smooth gravel floor.

Our drive around the island took us along white sand beaches at the base of sheer, thousand-feet cliffs beside the Gulf of Aden. Over the mountain passes we had views of alternating jagged mountains and rolling forest-covered hills, and along the tops of razor-sharp limestone buttresses at the edge of the cliff-backed coastal plain called the Noged, which stretches along most of the south coast of the island. While travelling across the mountainous centre of the island from the north coast to the south we stopped for a break at a deep freshwater pool set between the steep sides of a wadi. Cooling off in the water, we found dozens of Socotrean freshwater crabs - another endemic species.

By the end of the second week we had visited more than 25 sites in this slow and bone-jarring manner, and we all looked forward to our last week, which was to be spent aboard a dhow, while we visited the three smaller islands.

This part of the Indian Ocean has seasonal monsoons, which aren't so much periods of heavy rain as of strong winds. This accounts, in part, for the isolation of the islands, and the importance of the proposed new harbour - without it there is no way to get on or off the islands safely for several months of the year. Just one month later and our boat trip would have been very uncomfortable or even impossible.

The dives were very exciting: every time we got in the water, we knew it was at a site which had never been explored before. Because the islands owe their existence to the presence of a shallow ridge extending eastwards from Somalia, for the diver there are no spectacular drop-offs like those of Sinai, or many oceanic islands. For me, this was more than compensated for by the pristine nature of the underwater wilderness here. Most sites we visited were characterised by a gently sloping or almost flat sea-bed - the greatest depth I dived to throughout the whole trip was 20m.

The north coasts of all four of the islands are characterised by spectacular and extensive areas of hard and soft corals, and particularly of expanses of huge plates of branching Acropora corals (the largest I measured was well over 4m across). Several sites had incredibly diverse communities of beautiful soft corals, and everywhere we went there were huge numbers of fish, large and small. Part of my job was taking a sort of census of key fish groups, but at two of our Abd-al-Kuri sites there were so many big fish, and they were so unafraid, I couldn't see more than a few metres in any direction.

The worst culprits were surgeonfishes, in particular the eyestripe surgeon, Acanthurus dussumieri. Almost everywhere there were equally curious groupers, mostly the tomato grouper, Dermatolepis striolaatus. Nowhere else have I ever seen six groupers, each over a metre long, sitting side by side in an area the size of a billiard table.

For me, our most spectacular dive was at one of the few places we visited on the south side of any of the islands. Diving at the base of a vertical limestone cliff we found a surreal landscape of caverns, chasms, cliffs and sculpted rocks, all covered with an amazing array of algae, worms, sea squirts and corals. Through this landscape drifted, swam and darted fish of all shapes and sizes.

One of the noticeable aspects up to this point was the lack of sharks. I could have kicked myself when we surfaced and a colleage said: 'Did you see those three sharks which swam out behind you as you went past the big cave?'

A possible explanation for the large numbers of fish close to shore here is that there is almost no fishery for inshore fish at any of these islands. Local villagers catch what they need for their personal consumption, and no more. But this situation is likely to change quickly, in particular for relatively high value species of shallow water animals, such as the rock lobster. In a classic demonstration of unsustainable fisheries, the lobster population of nearby Oman was devasted in the 1980s; that of the mainland of Yemen has been decimated in the past three or four years. Now the lobster boats are looking for other sources, with Socotra the obvious target. We saw the evidence in what was perhaps the first of many mainland lobster boats anchored off the coast of Socotra.

The main cash income for the fishermen of the three inhabited islands (Darsa is uninhabited) is almost exclusively from the export of dried shark and sharkfins to the mainland, where the meat is eaten locally and the fins re-exported for the lucrative far eastern trade. This may account for the lack of sharks. I saw just one very small black tip. Everywhere we went, there were dried shark remains on the beach (some of them very big beasts I'm glad not to have met in the water). There is anecdotal evidence that even here, at one of the least visited sites anywhere, the shark fishery is on the verge of collapse.

The fishermen told us that their catches have declined over the past year or two, which is often an early sign of serious overfishing. A collapse in the shark fishery here would not only be a disaster for sharks, but also for the coastal communities of the islands, who currently have little else to fall back on. Almost as common on the beaches as shark remains were those of turtles, which are often used for food when caught accidentally, but are not deliberately hunted. There was evidence of turtle nesting sites on the islands, which may be of global importance, and certainly have regional significance.

By the end of the three weeks' work, I had lost more than a stone, been hungry, thirsty, sunburned, exhausted and sick by turns - and yet visiting the Socotrean islands was the most exciting and interesting trip I have ever been lucky enough to make. We found an amazing variety of marine habitats, from kelp forests to coral reefs, all in a virtually untouched condition almost unheard of in many parts of the world. The first signs of potential problems are, however, beginning to appear, and finding ways to help the people of the islands to improve the quality of their lives through increased education, health care and development, without threatening the future of biological diversity here is now a priority.

The islands are one of the very few places left anywhere which are so nearly pristine, both below water and above, and yet that could be destroyed in the space of a very few years by uncontrolled or careless development. Perhaps this time the changes can be made in a way which means that one of the most beautiful and intriguing places on earth is preserved for the future. The price of failure will be yet another unremarkable and barren group of islands which will only be remembered as having been something very special, once upon a time.