Puerto Rico is an island which is the top of a submerged mountain crest. The Atlantic Ocean reaches its greatest depth, 27,992 feet, in a chasm about forty-five miles north of Puerto Rico, known as the Milwaukee Deep. South of the island, only a short distance away, the Caribbean Sea reaches a depth of 12,000 feet. There are three small satellite islands, Vieques, Culeha, and Mona, whose combined area when added to the main island, give Puerto Rico a territory of 3,423 square miles, or approximately 2,000,000 acres. Almost three-fourths of the island of Puerto Rico lies in the mountainous interior, which has elevations up to 4,400 feet. The mountains slope down to a coastal plain which varies in width from eight to thirteen miles to the north and from two to eight miles to the south. The plain is chiefly devoted to the raising of sugar, the central crop in the island’s economy. Here is where the majority of the population is concentrated. The coastal climate is semi-tropical, with a mean temperature of 73.4 degrees Fahrenheit in the winter and 78.9 degrees Fahrenheit in the summer. In San Juan the average number of annual hours of sunshine is 2,845 (7.8/day), which indicates a high level of solar radiation. The climate is moderated by the ocean breezes. Mountain temperatures average only five to ten degrees lower than coastal, and variations from these mean temperatures are slight. The breeze blows from the sea during the day, and at night the coastal plains are somewhat cooled with air moving down from the mountains. In spite of the little difference in mean annual temperature between winter and summer, the seasons are fairly well marked in terms of crops, rainfall, and the duration and humidity of the winds.
The distribution of individuals on the island is patterned by the geographic features, with the coast being more populated than the interior. Anthropological papers have shown the biological and physiological differences stemming from environmental sources.