n ex-military area might not sound the ideal candidate for an eco-tourism venture, but Chumbe Island's former out-of-bounds status has served it well ecologically. Although only 12km from Zanzibar's Stone Town, the island was spared the tourist development that has caused so much damage elsewhere in the archipelago. Instead, Chumbe's coral is flawless and supports dense concentrations of marine life. Thanks to the efforts of Sybille Riedmiller, a German-born sociologist who has long lived in Tanzania, there has been no fishing within its waters for seven years. Spotting her opportunity, Riedmiller bought Chumbe's lease in 1992. She built seven eco-bungalows and has used revenue from tourism to fund conservation and research on the island, and to teach neighbouring communities the importance of managing the reef as a sustainable resource. In 1994 Chumbe became Tanzania's first marine reserve and has since won UN Protected Area and World Heritage status, as well as several travel industry awards.

To visitors, it offers the ultimate in escapism, a private desert island on which you can remove yourself mentally, should you choose, from the eco-campaign's front line while physically remaining part of it. Alternatively you can immerse yourself in eco-info and boost the feel-good factor born of knowing your presence is doing good. Both are indecently pleasurable. Snorkelling above the 200 species of coral is dazzling, or you can scuba dive the 16m-deep coral wall among 370 enticingly-named fish species: Moorish idols, Oriental sweetlips, butterfly-fish. Dolphins, turtles, lobsters and a resident metre-long grouper also make appearances. At low tide you can walk round the island or explore a large intertidal pool overgrown with mangroves and sheltered by huge baobabs.

Behind the palm-lined beaches lies an interior of rare coral rag forest, which you can explore alone or with a guide. The greenery seems rainforest-lush, but these are species specially adapted to life with little water or humus. Monstrous Coconut crabs, the largest land crabs at up to 45cm in diameter, crack open their favourite food with vice-like claws, while a WWF-sponsored programme monitors Chumbe's population of endangered Ader's duiker. From the lighthouse, built in 1904, you can survey turquoise-streaked waters dotted timelessly with dhows.

The lighthouse-keeper's house now forms Chumbe's visitor centre, where local schoolchildren come to learn reef maintenance before enjoying a snorkel. The island's six rangers are former fishermen themselves; part of their job is to teach neighbouring communities about the reef's endless potential if properly respected as a sustainable resource.

Generous Swahili feasts, combining Arabic, Indian and African influences, are perhaps best digested swinging in the hammock by your bungalow (made from a recycled fishing net). Built from natural materials, the palm-thatched bungalows are decorated with traditional African fabrics and artwork. Beds raised on mangrove poles let you gaze out to sea at night.

But behind the simplicity of handmade wooden furniture and driftwood shelves are the latest environmental devices. Chumbe lacks fresh water, so the roofs are designed to collect rainwater during the wet season, which is then filtered into a cistern beneath the floors and pumped up through a solar-powered heating system into the showers. Used shower water is recycled through beds planted with thirsty species, such as banana and papaya, which thrive on the minerals from soap. Compost toilets preserve water and ensure that no waste seeps from septic tanks onto the reef (mixed with kitchen compost, human waste quickly decomposes to valuable fertiliser). Lighting is also solar-powered.

The guest book confirms how enjoyable it is to visit Chumbe and how privileged visitors feel. "Nothing could have prepared us for this beauty," writes one guest. "A beacon of hope for tourism in Africa," declares another.

On the island's eastern side, salt spray prevents anything from growing, but 15,000-year-old fossilised coral and giant clams testify to its past as a living reef. Thanks to Reidmiller, Chumbe's untarnished future has also been secured.