Extremely isolated, Aldabra is almost untouched by humans. Aldabra atoll is closer to the coast of Africa 630 km (390 mi) than to Mahé, and is in the most southwesterly part of the Seychelles. It is 407 km (253 mi) northwest of Madagascar and 440 km (270 mi) from Moroni on the Comoro Islands. The atoll is the largest raised coral reef in the world with an elevation of 8 metres (26 ft); and the second-largest atoll in the world after Kiritimati Atoll. Located at 9°24′S 46°22′E / 9.400°S 46.367°E / -9.400; 46.367 and belongs to the Aldabra Group, one of the island groups of the Outer Islands of the Seychelles, which includes the island of Assumption and the atolls of Astove and Cosmoledo. Aldabra atoll is 34 kilometres (21 mi) long (in east-west direction) and 13 kilometres (8.1 mi) wide. It has a large shallow lagoon,196 square kilometres (76 sq mi) in area, of which roughly two thirds is dry during low tide. The lagoon is encircled by fringing coral reef. Around the rim of the lagoon are the larger islands of the Atoll. The total land area of the Atoll is 155.4 square kilometres (60.0 sq mi). The size including the lagoon is 380 square kilometres (150 sq mi). The outside rim of the atoll has three passages which connect to the lagoon, which is 6–10 kilometres (3.7–6.2 mi) in width as it opens to the sea. The depth of water in the lagoon averages about 5 metres (16 ft); however, the passages that open to the sea are up to 20 metres (66 ft) deep and strongly affected by tidal currents.
Aldabra atoll has, besides the four larger islands, some 40 smaller islands and rocks, all inside the lagoon, as well as a few very small islets at the West Channels between Grand Terre Island and Picard Islands, the largest of those being Îlot Magnan.
More Islands (unspecified location, but sizes are included under "Other Islands":
The atoll reflects both fossil and geomorphological features, the former is the source of the biodiversity seen today. The atoll is made of reef limestone of Pleistocene age (with irregular coral formations called "champignon", made up of two layers of varying stages of crystallization ) and this extends over an average width of 2 kilometres (1.2 mi) rising to a height of 8 metres (26 ft) above sea level, and forming the rim line (low cliffs with "deep notches, preceded by jagged pinnacles") of the shallow central lagoon. Geologically the limestone beds have been subjected to striation, sink holes and pits with prominent and continuous limestone bed on the eastern side above the sediment deposits. The coastline has undercut limestone cliffs above a perched beach; it is in two clear terraces of 8 metres (26 ft) and 6 metres (20 ft) height above sea level. Sand dunes dominate the windward south coast. While the terrestrial topography (spread over an elevation range of 0–8 metres (0–26 ft)) is rugged and dictated by the geomorphic conditions, the land surface comprises limestone of about 125,000 years age, which has uplifted many times above the sea level. The surface conditions are criss-crossed and riddled with pot holes and pits. In the eastern zone of the lagoon, though the surface is continuous, sediment beds are also seen. The windward southern coast is made up of sand dunes.
Aldabra is situated in the dry zone of the south-west Indian Ocean. The northwest monsoon is from November to March and brings the heaviest rainfall. In the remaining months, the south-easterly trade winds are recorded. Aldabra receives an average of 960 millimetres (38 in) rainfall per year. Cyclones are rare in the Seychelles on account of its nearness to the Equator. Tides in the coastal zone rise to 3 metres (9.8 ft) height, causing channel currents, and a huge influx of water; the main channel drains 60% flow into the lagoon. The reported monthly mean maximum temperature recorded in December is 31 °C (88 °F). The mean minimum temperature recorded in August is 22 °C (72 °F).
The earliest study of the flora and fauna, and also the geomorphological structure is dated to 1910. There are 307 species of animals and plants on Aldabra. Reptiles are the prominent terrestrial fauna. Sir David Attenborough called Aldabra "One of the wonders of the world", and it is also known as one of "crown jewels" of the Indian Ocean.
The higher areas of Aldabra are covered in pemphis, a thick coastal shrub, while the lower areas, which are home to the giant tortoises, are a mixture of trees, shrubs, herbs and grasses. There have been recorded 273 species of flowering plants, shrubs, and ferns on the atoll. There are dense thickets of Pemphis acidula, and a mixture of grasses and herbs called "tortoise turf" in many areas. This flora includes 19 endemic species and 22 species that are only common to neighboring islands, and several of these species are on the IUCN Red List. The tropicbird orchid (Angraecum seychellarum) is the national flower of Seychelles and is found in the dry craggy limestone champignon of Aldabra. Other endemic plants such as Pandanus aldabrensis, the Aldabra lily (Lomatophyllum aldebrense) and a sub-species of tropicbird orchid, Angraecum eburneum. The lagoon is bordered by mangrove forests, and has large inland seagrass meadows as well as areas of coral reef and sand flats. The mangroves, which thrive in tidal mudflat areas and saline conditions, are seen on the shores of the lagoon and are integral to the coastal ecosystem. There are seven species of mangrove occurring on Aldabra, three of which are rarely occurring species. These include 'Mangliye blan' or white mangrove (Avicennia marina) which grows to 12 metres (39 ft), 'Mangliye lat' or black mangrove (Bruguiera gymnorhiza) which grows to 18 metres (59 ft) in a conical shape, 'Mangliye zonn' (Ceriops tagal) which grows to 7 metres (23 ft) with a buttressed trunk, and 'Mangliye rouz' or red mangrove (Rhizophora mucronata) which is the tallest species up to 20 metres (66 ft) in height.
The atoll has distinctive fauna including the largest population of giant tortoises (Aldabrachelys gigantea) in the world (100,000 animals). Tortoise size varies substantially across the atoll, but adult tortoises typically have a carapace length of about 105 centimetres (41 in) and can weigh up to 350 kilograms (770 lb). They are herbivores and feed on a variety of plants, trees and even algae that grows in the freshwater pools. The tortoises mate between February and May, the females then lay their eggs from June to September in areas with suitable soil layers. They lay eggs (the size of golf balls ) in a clutch of three to five eggs every few years in high-density areas and 14-16 eggs in low-density areas. The females can lay several clutches in a year and the incubation period is 73–160 days. The small vulnerable juveniles have to survive the predation by coconut crabs, land crabs, rats and birds. In the past giant tortoises have been relocated to many other islands in Seychelles and also to Victoria Botanical Gardens in Mahé. One of the longest-lived Aldabra giant tortoises was Adwaita, a male who died at the age of about 250 years at Kolkata's Alipore Zoological Gardens on March 24, 2006.
Aldabra is a breeding ground for the hawksbill sea turtle (Eretmochelys imbricata) and green sea turtle (Chelonia mydas). Aldabra has one of the largest populations of nesting green turtles in the Western Indian Ocean. Aldabra has a large population of the world's largest terrestrial arthropod, the coconut crab (Birgus latro); and hosts the white-throated rail, the only surviving flightless rail species in the Indian Ocean. Sharks, manta rays, barracuda can be found in the seas surrounding the island. During the Pleistocene the dominant land predator was the crocodilian Aldabrachampsus which is now extinct. Three extant species of lizards occur, the skink Cryptoblepharus boutonii and the geckos Phelsuma abbotti and Hemidactylus mercatorius. Pleistocene fossils also indicate the former presence of an Oplurus iguana and other skink and gecko species. There are three endemic species of bat from Aldabra: Paratriaenops pauliani, Chaerephon pusilla and the Aldabra flying fox (Pteropus aldabrensis), as well as the more widely distributed Mauritian tomb bat (Taphozous mauritianus). There are 1,000 species of insects, many of them endemic. Many species of butterflies also flutter around Aldabra.
Endemic birds include the Aldabra drongo (Dicrurus aldabranus), the Aldabran subspecies of the white-throated rail (Dryolimnas cuvieri aldabranus), the last surviving flightless bird of the Indian Ocean region, and the endemic Aldabra fody (Foudia aldabrana), The islands are important breeding grounds for thousands of seabirds, including several species of tern, red-tailed tropicbirds, white-tailed tropicbirds, red-footed boobies, and the world’s second largest breeding populations of great and lesser frigate birds. The bird fauna is most similar to Madagascar or Comoros and other birds found here include greater flamingos, the Madagascar pond heron, Comoros blue pigeon, Madagascar kestrel, Madagascar coucal, Madagascar nightjar, Madagascar bulbul and souimanga sunbird.
At least 13 species of cetaceans including dolphins, orcas, and especially humpback whales have been known in the waters. Dugongs, once thought to be regionally extinct in the 18th century, have been confirmed for multiple times in very recent years.
Conservationists feared a major threat to the atoll's biodiversity in the 1960s when, as part of the British Indian Ocean Territory, the British made plans to set up a military establishment on the atoll. Due to national and international opposition this plan was cancelled in 1967. This incident became known as the 'Aldabra Affair' in England. Invasive alien species such as rats, cats and goats that were introduced in the past threaten the native biodiversity of the atoll. Goats were eradicated from the atoll in 2012 after a long-term eradication program. Cats have been removed from all of the islands except Grande Terre Island, which allowed for the reintroduction of the Aldabra rail to Picard Island. Research into a feasibility study to eradicate rats from the atoll has been undertaken. Aldabra was until recently free of introduced birds but unfortunately the introduced Foudia madagascariensis that was introduced to Assumption Island, now occurs on Aldabra. an eradication program for this bird on both Assumption and Aldabra is almost done. Due to the limited space of its habitat, extreme weather conditions, epidemic and limited range could also pose serious threats to the entire ecology of the atoll.
Aldabra atoll was designated a UNESCO World Heritage Site on 19 November 1982. it is one of the two UNESCO World Heritage Sites in the Seychelles ;, and is managed and protected by the Seychelles Islands Foundation (SIF). The marine protected area extends 1 kilometre (0.62 mi) into the sea to ensure preservation of its marine fauna. Eco tourism is controlled and introduction of invasive species is restricted. Based on the evaluation process, UNESCO inscribed the site, a legally protected special reserve of 35,000 hectares (86,000 acres), on the list of World Heritage Sites under three criteria: Criterion (vii): Aldabra Atoll encompasses a large expanse of relatively untouched natural beauty where a number of important animal species and some plant species thrive, along with remarkable land formations, and its process provides a unique spectacle of natural phenomena; Criterion (ix): The atoll is a superlative example of an oceanic island ecosystem in which evolutionary processes are active within a rich biota. The size and morphological diversity of the atoll has permitted the development of a variety of discrete insular communities with a high incidence of endemicity among the constituent species that are typical of island ecosystems. The natural processes take place with minimal human interference and can be clearly demonstrated in their full complexity; and Criterion (x): Aldabra provides a natural laboratory for the study of the process of evolutionary ecology and is a platform for key scientific discovery. The atoll constitutes a refuge harboring viable populations of a range of rare and endangered species of plants and animals. These include the last giant tortoise and flightless bird populations of the Western Indian Ocean, a substantial marine turtle breeding population, and large seabird colonies which number in the tens of thousands. The substantial tortoise population is self-sustaining and all the elements of its inter-relationship with the terrestrial environment are evident.
BirdLife International declared Aldabra as an Important Endemic Bird Area (IBA) in 2001 on account of its large seabird colonies under categories A1, A2, A4i, A4ii and A4iii, covering an area of 33,180 hectares (82,000 acres) overlapping with the special reserve area of 35,000 hectares (86,000 acres) of Aldabra Atoll.
Aldabra became a Ramsar Wetland Site of International Importance in 2010. Covering 25,100 ha (over half the area of the whole atoll) the wetland ecosystem of Aldabra include the extensive shallow lagoon inside the atoll, which is carpeted with lush seagrass beds and patchy coral reefs, the intertidal mud flats, the coral reefs outside the lagoon, freshwater pools, beaches, and 2000 ha of mangrove stands. These wetlands support several endangered species including the increasing number of turtles at the atoll, dugongs and many other bird, fish and invertebrate species.
Aldabra was designated as a site under the Indian Ocean South East Asia (IOSEA) turtle network, in their 2014 convention.
A small scientific research station of the SIF is based on La Gigi village on Picard Island. The permanent staff (currently 12) conduct research to study Aldabra's biodiversity.
Aldabra is not easily accessed. No airstrips, helipads or landing jetties have been permitted to be built on the atoll. The nearest airfield on Assumption is 50 kilometres (31 mi) south-east of Picard Island. Supply ships operating from Mahé provide food and other essentials once every two months to the scientists and staff at the research station.
Cruises are operated by several companies along with dive boats which may visit the atoll on expedition tours. Visits to the island by people other than the scientists and staff of the SIF are strictly controlled and only guided tours are provided with prior permission. As of 2012, an average of 900 tourists visit the atoll a year. Within the atoll, paved walking paths exist from the village of La Gigi, which leads to a promontory from where scenic views of the large lagoon (during low tides) and the mangrove species are seen.