Chira Island is a 3,000-hectare (7,400-acre) Costa Rican Pacific island located at the upper end of the Gulf of Nicoya, an extensive inlet of sea water that is the result of a geological fault that has caused the land to submerge, leaving exposed only the tops of what were formerly low hills. These are the various islands that dot the gulf, the largest of which being the Island of Chira - Costa Rica's second largest island. The Gulf of Nicoya separates Guanacaste from mainland Costa Rica and is fed by the Tempisque River.