Pedro Mascarenhas was the first to explore the island atoll that comprises what landform?
Pedro Mascarenhas:  He was the first European to discover the island of Diego Garcia in the Indian Ocean in 1512. Diego Garcia: 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.
Chagos Archipelago