The atoll is com­prised of four large coral islands which enclose a shal­low lagoon, the group of islands is itself sur­round­ed by a coral reef. Due to dif­fi­cul­ties of access and the atoll‘s iso­la­tion, Aldabra has been pro­tect­ed from human influ­ence and thus retains some 152,000 giant tor­tois­es, the world‘s largest pop­u­la­tion of this rep­tile.