Cayo Santiago
Cayo Santiago, also known as Monkey Island, is an uninhabited island, 950 m (0.6 mi) southeast of Punta Santiago, Humacao, Puerto Rico. It extends 600 m (656 yd) north-south and 400 m (437 yd) east-west, including the flat northeastern peninsula which is connected to the main part only by a narrow sandy isthmus. 600 meters west of the southermost point is a shoal, Bajo Evelyn, which has a least depth of 8 fathoms. While the island is flat in the north, it reaches a height of 34.9 m (104.5 ft) 2.9 km (1.8 mi) southwest of the port, in a small rocky hill called El Morrillo, which rises abruptly from the water and the lowland around it. The area of the island is 0.14 km² (139,179 m²: Census block 2000, Census Block Group 2, Census tract 1801, Humacao Municipio, Puerto Rico), of which the northwestern peninsula accounts for about 0.02 km². The island is part of Punta Santiago barrio of Humacao.
The island has a free ranging population of about 950 Rhesus monkeys. The monkeys are offspring of an original group of 409 monkeys imported from India that were used for scientific research in 1938. Today the island serves as a research center for the University of Puerto Rico (Caribbean Primate Research Center), the National Institutes of Health, and Harvard University. The station accommodates 11 researchers and technicians who take the short boat trip from Humacao on a daily basis and normally don't stay on the island.
Cayo Batata, the second island belonging to Humacao, is located 5.7 km to the southwest.