Mona has an area of about 57 km2 (22 sq mi) and
lies 66 km (41 mi) west of the main island of Puerto
Rico, 61 km (38 mi) east of the Dominican Republic,
and 49 km (30 mi) southwest of Desecheo Island,
another island in the Mona Passage.
Mona has been designated an ecological reserve by
the Puerto Rican government and is not permanently
inhabited. The US census of 2000 reports six housing
units, but a population of zero. The island is a ward
(barrio) of the municipality of Mayagüez, together
with Monito Island 5 km northwest (Isla de Mona e
Islote Monito barrio). This is the largest ward of
Mayagüez by area, and the only one without
permanent population. The total land area of both
islands in the barrio is about 56.93 km² (Mona Island
56.783 km² and nearby Monito Island 0.147 km²), and
it comprises 28.3 percent of the total land area of the
municipality of Mayagüez. Desecheo Island, 49 km to
the northeast, is part of Sabanetas barrio.
Mona is a mainly flat plateau surrounded by
sea cliffs. It is composed of dolomite and
limestone with many caves found throughout.
With an arid climate and untouched by
human development, many endemic species
inhabit the island, such as the Mona Ground
Iguana (Cyclura cornuta stejnegeri). Its
topography, ecology, and modern history are
similar to that of Navassa Island, a small
limestone island located in the Jamaica
Channel, between Jamaica and Haiti.