Historically, the name Aldabra was Al-Hadra or Al-Khadra (with several variants), given by Arab seafarers for "the atoll’s harsh, sun-baked environment"; this name was included in the Portuguese maps of the 16th century. They had named the Indian Ocean as Bahr-el zanj. It was visited by Portuguese navigators in 1511. The islands were already known to the Persians and Arabs, from whom they got their name. In the middle of the 18th century, the atoll became a dependency of the French colony of Réunion, from where expeditions were made for the capture of the Aldabra giant tortoises. As there are no surface freshwater sources on Aldabra, the interests of the explorers (no proof of any explorer's visit prior to 1742) was only to exploit the species of tortoise, turtle and fish, and not to inhabit the atoll. In 1810, with Mauritius, Réunion, the Seychelles and other islands, Aldabra passed into the possession of Great Britain. Réunion was later returned to France, and Mauritius gained possession of Aldabra as well as the rest of the Seychelles. The previous inhabitants were emigrants from the Seychelles.