Census Bureau map
|Location||Humacao, Puerto Rico|
|Area||0.139179 km2 (0.053737 sq mi)|
|Length||0.6 km (0.37 mi)|
|Width||0.4 km (0.25 mi)|
|Highest elevation||34.9 m (114.5 ft)|
|Highest point||El Morrillo or "Big Cay hill top"|
|Pop. density||0 /km2 (0 /sq mi)|
The island measures approximately 37.5 acres (15.2 ha), 600 metres (660 yd) north–south and 400 metres (440 yd) east–west, including a "Small Cay" which is connected to the main part ("Big Cay") by a narrow sandy isthmus. Six hundred meters west of the southernmost point is a shoal, Bajo Evelyn, which has a shallow depth of 8 fathoms. While the island is flat in the north, it reaches a height of 34.9 m (114.5 ft), 2.9 km (1.8 mi) southwest of the island's port, on 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 (1,498,110 sq ft): Block 2000, Census Block Group 2, Census tract 1801, Humacao Municipio, Puerto Rico), of which the northeastern peninsula accounts for about 0.02 km2 (0.01 sq mi). In the late 1940s, the island was expropriated by Puerto Rico from its private owners and ceded to the University of Puerto Rico School of Medicine when it was established [for details consult external links].
Since December 1938, the island has been the habitat for a free-ranging population of Rhesus monkeys. The monkeys are the offspring of an original group of 409 monkeys imported from India by Clarence R. Carpenter and the School of Tropical Medicine in San Juan that was operated by Columbia University College of Physicians and Surgeons and the University of Puerto Rico. Today, the colony, which numbers over a thousand animals, serves as a research resource supported by the National Institutes of Health and the University of Puerto Rico Caribbean Primate Research Center for investigators from many institutions in the USA and several in Europe. A pictorial history on the 75 years of the colony was recently published [see external link].
Cayo Batata, a small island located 5.7 km (4 mi) to the southwest of Cayo Santiago, is under the jurisdiction of Humacao and is uninhabited by humans.