Question: Pedro Mascarenhas was the first to explore the island atoll that comprises what landform?
Context: The Mascarene Islands ( ) or Mascarenhas Archipelago is a group of islands in the Indian Ocean east of Madagascar consisting of Mauritius, Réunion and Rodrigues.  The collective title is derived from the Portuguese navigator Pedro Mascarenhas, who first visited them in the early sixteenth century.  The islands share a common geologic origin in the volcanism of the Réunion hotspot beneath the Mascarene Plateau and form a distinct ecoregion with a unique flora and fauna. Butaritari is an atoll in the Pacific Ocean island nation of Kiribati.  The atoll is roughly four-sided.  The south and southeast portion of the atoll comprises a nearly continuous islet.  The atoll reef is continuous but almost without islets along the north side.  Bikati and Bikatieta islets occupy a corner of the reef at the extreme northwest tip of the atoll.  Small islets are found on reef sections between channels on the west side.  The lagoon of Butaritari is deep and can accommodate large ships, though the entrance passages are relatively narrow.  It is the most fertile of the Gilbert Islands, with relatively good soils (for an atoll) and high rainfall.  Butaritari atoll has a land area of 13.49 km² and a population of 4,346 as of 2010 .  During World War II, Butaritari was known by US forces as Makin Atoll, and was the site of the Battle of Makin.  Locally, Makin is the name of a separate atoll three kilometers to the northeast of Butaritari. Olhuala is a type of yam grown on the island atoll of Fuvahmmulah, Maldives ("olhuala" is a word from the Dhivehi language). North Keeling is a small, uninhabited coral atoll, approximately 1.2 km2 in area, about 25 km north of Horsburgh Island.  It is the northernmost atoll and island of the Australian territory of the Cocos (Keeling) Islands.  North Keeling comprises just one C-shaped island, a nearly closed atoll ring with a small opening into the lagoon, about 50 m wide, on the east side.  The lagoon is about 0.5 km2 in area.  The island is home to the only surviving population of the endemic, and endangered, Cocos buff-banded rail, as well as large breeding colonies of seabirds.  Since 1995, North Keeling Island and the surrounding sea to 1.5 km from shore have been within the Pulu Keeling National Park. Operation Flintlock was the campaign against the Japanese in the Marshall Islands in the Pacific theatre of World War II, from 31 January to 4 February 1944.  The operation involved the invasions of Kwajalein, Eniwetok, and Majuro atolls.  Admiral Nimitz, Commander of the U. S. Pacific Fleet, chose two islands in Kwajalein Atoll, Roi-Namur Island and Kwajalein Island, as primary targets in the U. S. invasion of the Marshall Islands.   Kwajalein Atoll contained communication and weather observation units and two Japanese airstrips on Roi-Namur and Kwajalein Islands, a seaplane base situated at Ebeye Island, a submarine base at Roi-Namur Island, and other Japanese installations scattered on various islands throughout Kwajalein atoll.   Kwajalein atoll, particularly Roi-Namur and Kwajalein Islands, were subjected to heavy bombardment.  This attack also sank a large number of Japanese ships in Kwajalein Lagoon.  Bitter fighting between Japanese forces and the U. S. 4th Marine Division on Roi-Namur, and the U. S. 7th Infantry Division on Kwajalein, resulted in a U. S. victory on 4 February 1944.  The attack of the Japanese in the Marshall Islands was the first US attack, and capture, of Japanese territory, since the land was held by Japan before the start of World War II.  The capture of Kwajalein Atoll during Operation Flintlock provided American forces with a base of operations that assured the recapture of the Philippines and eventually the fall of Japan. Dom Pedro Mascarenhas (1470 – 16 June 1555) was a Portuguese explorer and colonial administrator.  He was the first European to discover the island of Diego Garcia in the Indian Ocean in 1512.  He also encountered the Indian Ocean island of Mauritius in 1512, although he may not have been the first Portuguese explorer to do so; earlier expeditions by Diogo Dias and Afonso de Albuquerque along with Diogo Fernandes Pereira may have encountered the islands.  In 1528 explorer Diogo Rodrigues (after whom the island of Rodrigues is named) named the islands of Réunion, Mauritius, and Rodrigues the Mascarene Islands, after Mascarenhas. Jemo Island Atoll (Marshallese: Jemo̧ or Jāmo̧ , ) is an uninhabited coral island in the Pacific Ocean, in the Ratak Chain of the Marshall Islands north-east of Likiep Atoll.  The island is oval-shaped, and occupies the southwestern end of a narrow submarine ridge that extends to the northeast for several kilometers.  Its total land area is only 0.16 km2 .  The island is traditionally held as a food reserve for the family of Joachim and Lijon deBrum, passed down to Lijon debrum from Iroijlaplap Lobareo and is owned by the current Likiep land-owning families of Joachim and Lijon debrum, grandkids of Iroijlaplap Jortõka of Ratak Eañ. Diego Garcia is an atoll just south of the equator in the central Indian Ocean, and the largest of 60 small islands comprising the Chagos Archipelago.  It was settled by the French in the 1790s and was transferred to British rule after the Napoleonic Wars.  It was one of the "Dependencies" of the British Colony of Mauritius until it was detached for inclusion in the newly created British Indian Ocean Territory (BIOT) in 1965.  Between 1968 and 1973, the population was forcibly removed by the United Kingdom and the United States (in order to establish an American base) through intimidation of locals and denying the return of any who left the island.  Many were deported to Mauritius and Seychelles, following which the United States built a large naval and military base on Diego Garcia, which has been in continuous operation ever since.  s of 2015 , Diego Garcia is the only inhabited island of the BIOT; the population is composed of military personnel and supporting contractors. D. Diogo Rodrigues, D. Diogo Roiz ( 1490-1496; Lagos, Portugal – 21 April 1577; Colvá, Gôa) was a Portuguese explorer of the Indian Ocean who sailed as an ordinary helmsman under the command of D. Pedro Mascarenhas around Goa.  They sailed from the Cape of Good Hope eastward into little-known waters of the newly discovered route to Gôa.  It was after him that the island of Rodrigues was\is named between 4 and 9 February 1528 after discovering it during his only return journey from Gôa via Cochin (left on 15 January 1528) to Portugal, following which he rose to the rank of a knight "(cavaleiro)".  He then returned to Gôa and made a mark in the history of the Portuguese empire in the subcontinent around the mid 16th century. The Aldabra giant tortoise ("Aldabrachelys gigantea"), from the islands of the Aldabra Atoll in the Seychelles, is one of the largest tortoises in the world.  Historically, giant tortoises were on many of the western Indian Ocean islands, as well as Madagascar, and the fossil record indicates giant tortoises once occurred on every continent and many islands with the exception of Australia and Antarctica.  Many of the Indian Ocean species were thought to be driven to extinction by over-exploitation by European sailors, and they were all seemingly extinct by 1840 with the exception of the Aldabran giant tortoise on the island atoll of Aldabra.  Although some remnant individuals of "A. g. hololissa" and "A. g. arnoldi" may remain in captivity, in recent times, these have all been reduced as subspecies of "A. g. gigantea".
Answer: Chagos Archipelago