Randall, who visited the atoll during an expedition by the Smithsonian Institution in 1975. Acadia Islet, along the atoll's north and east rim, is several times larger than the other three islets combined, measuring . Very long and thin, the islet is largely forested and is composed of ridges of coral rubble. It is named after the Acadia, a ship that was wrecked on Ducie in 1881. Pandora Islet, in the south, is the second largest. It is composed of sand and coral rubble that borders the lagoon. It is named after HMS Pandora. Edwards Islet lies immediately to the east of Pandora Islet and has the same characteristics. It is named after Edward Edwards, captain of HMS Pandora. Westward Islet, west of Pandora Islet, is the smallest. It appears sandy from a distance, but the soil is composed of coral rubble and dead shells. Its highest point rises above average sea level. It is named after the Westward, the ship that carried the members of the National Geographic Society and the Oceanic Institute during their 1970–71 expedition. The atoll has a central lagoon, accessible by boat only by way of a channel wide located in the southwest, between Pandora and Westward Islets. It has a maximum depth of and its bottom consists of sand and coral. Whirlpools in the lagoon are common, caused by caves that drain the water from the lagoon into the ocean. Pandora is known as being one of the three coastline vertices for Point Nemo, the set of coordinates in the South Pacific Ocean that represents the furthest point from any land in three directions. Flora The vegetation in the atoll is sparse, because of the lack of fresh water. Only two species of vascular plant are currently known to grow there – one of the smallest such floras on any island. Acadia, Pandora and Edwards Islets are forested with Heliotropium foertherianum, but Westward Islet is not. Pemphis acidula has also been recorded on Ducie; specimens were found during an expedition in 1991. During the expedition of Hugh Cuming in 1827 and the 1922 Whitney South Sea Expedition, Lepturus grass was found on Acadia Islet. However, it disappeared when storm waves deforested the island some time before the Smithsonian expedition of 1975. Thus H. foertherianum now dominates the vegetation of the islets. Additionally, there are a number of species of coralline algae, including Porolithon onkodes,