An atoll is a ring of coral that grew long ago around the edge of a sunken volcano. Aldabra is one of the biggest atolls in the world - a green ring of low islands surrounding a calm blue lagoon. From the air, it looks a bit like a giant footprint in the sea.
Almost no one lives on Aldabra. Just a small team of scientists and rangers - usually fewer than 20 people - who look after the wildlife. There is no airport. Visitors arrive by boat after several days at sea. Once on the island, you camp simply and walk softly.
The atoll is famous for its giant tortoises. There are around 100,000 of them, which is more giant tortoises than anywhere else in the world. They wander wherever they please, munching grass and resting in the shade. Sea birds nest by the thousand, and the lagoon is full of sharks, rays and turtles.
Aldabra is one of the few places left on Earth that looks almost exactly as it did before people arrived. Children in Seychelles learn about Aldabra at school - it is one of their country's biggest gifts to the planet. Scientists from all over the world come to study how a wild place works when it is properly left alone.