There's a tiny new island resort with a big mission: to preserve one of the world's most spectacular shallow-water reefs. Barracudas welcome

It's hard to get an island all to yourself unless you own one, as Richard Branson does--Necker Island, in the Caribbean--or you're rich enough to empty one out when you feel like it, as Bill Gates reportedly has done at a place called Mnemba Island Lodge, an astronomically expensive resort off the coast of Zanzibar in East Africa. I thought about Bill Gates quite a lot recently while I was staying on Chumbe, a tiny island, only half a mile long and 200 yards wide, also off the coast of Zanzibar, newly opened as a resort with rates in the merely stratospheric range. Local rumor had it that Gates was at Mnemba at the same time that I was at Chumbe. I kept wondering whether he could possibly be having a better time than I was as I snorkeled around the pristine coral reef off the island's shore or lounged with my boyfriend in our eco-bungalow built from mangrove poles and thatch.

It did seem unlikely that Gates had been required, as I had been, to wade knee-deep into the ocean to board the ferry for the 40-minute journey from Zanzibar. And when, one evening, a member of the Chumbe kitchen staff apologized to my boyfriend for burning the coconut rice we were supposed to be having for dinner, I found myself thinking, I bet they're not telling that to Bill Gates over on Mnemba. I was also not sure how Gates would feel about the ecologically correct toilets in the bungalows: instead of flushing, you toss a few scoops of earth into the bowl. It occurred to me that the richest man on the planet might prefer plumbing.

Chumbe leads a double life: it's not only a luxury resort, but also an important ecological preserve, home to one of the most spectacular shallow-water coral reefs in the world. The island is densely, steamily forested, and just a very small portion, next to the glistening white beach, has been claimed for human use. During much of the last century, Chumbe's sole human inhabitant was a lighthouse keeper; the keeper's quarters are now Chumbe's educational center and dining room. The lighthouse was abandoned in the early sixties, when Tanzania's newly formed government declared the channel between Zanzibar and Dar es Salaam off-limits to unauthorized traffic. As a result, fishermen in their dugout canoes rarely entered these waters. With fishing curtailed, the reefs in the channel flourished; but in the early nineties the fishermen returned, this time with outboard motors that gave them greater ability to follow the fish. In the ensuing years, Zanzibar's population doubled and tourism boomed, which meant that the channel was fished more aggressively, causing massive deterioration of the reefs along the Zanzibar coast.

The Chumbe reef would likely be seriously degraded were it not for German expatriate Sibylle Riedmiller, who has worked for decades in Tanzania and Latin America setting up environmental programs with overseas aid organizations. After the dispiritingly frequent experience of seeing projects collapse as soon as aid money stopped flowing, Riedmiller wanted to start a self-sustaining environmental project. When she saw Chumbe's reef in the early nineties, she realized it had great potential as an educational site--if it could be kept safe from the fishing industry.

Riedmiller lobbied the Tanzanian government, and in 1994 was given the rights to manage the island as a private company. "Actually," she says, "the government was interested in a tourism project. Conservation wasn't on the political agenda at all--much less marine conservation." But since then, Tanzanian schoolchildren have visited Chumbe to learn about the reef's role in protecting fish populations, and to walk the nature trail through the forest, where they can see coconut crabs scrambling up tree trunks and climb over coral rag, or fossilized coral. Riedmiller's organization also teaches fishermen about the ways the waters provide a breeding ground for fish that will eventually turn up in their nets farther along the coast. (All the outreach in the world doesn't prevent some locals from trying their luck: one day I saw a boatload of fishermen loitering near the reef; when approached by Chumbe rangers, they claimed, implausibly, that the wind had gone out of their sails.) Under Riedmiller's guardianship, the reef has flourished: fish, representing 380 species, swim thickly around, never having known the threat of the net. Ninety percent of the varieties of coral found off East Africa live there, and I am sure the other 10 percent would set up shop if they knew about it.

For the first years after Chumbe was reincarnated as an eco-zone, it accepted day visitors only; two summers ago the island opened to overnight guests. Typically, they either arrive for a few days at the end of a safari, or come for two weeks of utter peace and quiet. (There are no phones, faxes, or e-mail facilities on Chumbe; the place tends to attract well-off professionals in finance or publishing, as well as members of the rock-musician demographic.) Guests stay in seven bungalows with high, swooping roofs that look as if they had been devised by Robinson Crusoe in emulation of the Sydney Opera House. Actually, they were built by the Chumbe staff under the direction of German architect Jan Huelsemann. (Georg Fiebig, a German architecture student whom Riedmiller brought to the island in the mid nineties, designed the prototype.) The island has no groundwater, so each bungalow must be ecologically self-sustaining: rainwater is channeled from the roof to a cistern under the floor; solar panels heat the water and provide power for electric light. There's that composting eco-toilet, too. But the bungalows are unexpectedly stylish--open and airy, with mosaic floors (ours depicted a turtle) along with mangrove-pole couches, African-print cushions, and a big hammock. Each sleeping loft, reached by ladder, contains a mattress and mosquito netting and, as a bedside table, a low wooden bench that wouldn't be out of place in Wallpaper magazine.

Perhaps because Chumbe is primarily an ecological project, there's little of the fussy servility sometimes found at more conventional hotels. The place is run by a young English couple, Mark Johnson and Nicole Talbot, along with a dozen or so rangers and other staffers. We were there during Ramadan, which meant that none of the Tanzanian rangers could go snorkeling with us--getting water on their lips would have broken their fast--and so we were accompanied by a likable college-aged Englishman named Justin Andersen, who had spent much of the past eight months in Africa underwater. Justin looked as if he hadn't combed his hair since his arrival, and he also was foul-mouthed in a way that's common among those who have attended expensive British private schools: there were "shitloads of fish" to be seen, and, although the weather might be brilliantly sunny, it was perfectly capable of "starting to piss down" at any moment. Once Justin had shown us the reef's location--about a quarter-mile from shore--we were free to browse it any time we wanted. The huge, bulbous, veined forms of coral, in meaty shades of red and yellow, with occasional, unexpected outcroppings of brilliant blue, put me in mind of nothing so much as the internal organs and vessels of the human body--but as those organs must appear to a surgeon, which is to say full of beauty and wonder and fascination, and not in the least bit squeamish-making.

Mark and Nicole treated us as though we were just casual visitors to this really cool place they had found, rather than tourists on a pricey vacation. One evening, as my boyfriend and I were eating our supper of coconut-curried fish stew and breadfruit, Nicole, who looked rather like Sarah Jessica Parker and changed out of her shorts into a sundress for dinner every evening, rushed up to our table. "There's something going on by the jetty," she said. We put down our forks and ambled over in the dark, carrying flashlights. The beach was swarming with hundreds of hermit crabs, clambering frantically over rocks and over one another, turning the shore into a seething mass of legs and shells. Mating?Migrating?Shell shedding?No one could tell us what they were doing, but it was thrilling to watch. I don't know whether there are any crabs living on Mnemba, or if there are, whether anyone would interrupt Bill Gates's dinner to tell him about them; but if not, he's the poorer for it.

To get to Chumbe Island, you fly to Zanzibar and catch the ferry from the Mbweni Ruins Hotel near Stone Town. Avoid visiting during the rainy season, from April through June. Chumbe's bungalows, which sleep two, start at $200 per person, including all meals. For more information, contact Chumbe Island Coral Park, Zanzibar, Tanzania; phone and fax 255-54/231-040;