Located between the João de Arens beaches and the barra da Ria de Alvor, Gemeos Grandes (the "greater twins") is part of a collection of forty artificial reefs created by the Portuguese Institute for Sea and Atmosphere (IPIMAR) in 2000, placed in four rows at depths between 18m and 35m. They were meant to be used as a breeding and protection area for different local, marine species, and all along the reefs a gradual increase of biodiversity has been observed in the area.
The "greater twins" are made up of three blocks of concrete resting on the sand, at a depth of 30 meters, spaced about 30m to 40m apart, joined together by a cable guide. The most common species are morays, congers and zebra sea breams. Divers are advised to control their diving time, as the site can reach depths of up to 35 meters in its deepest area.