Geography

Miami and its suburbs are located on a broad plain between the Florida Everglades to the west and Biscayne Bay to the east, which also extends from Florida Bay north to Lake Okeechobee. The elevation of the area never rises above 40 ft (12 m)^[33] and averages at around 6 ft (1.8 m)^[34] above mean sea level in most neighborhoods, especially near the coast. The highest undulations are found along the coastal Miami Rock Ridge, whose substrate underlies most of the eastern Miami metropolitan region. The main portion of the city lies on the shores of Biscayne Bay which contains several hundred natural and artificially created barrier islands, the largest of which contains Miami Beach and South Beach. The Gulf Stream, a warm ocean current, runs northward just 15 miles (24 km) off the coast, allowing the city's climate to stay warm and mild all year.

Geology

View from one of the higher points in Miami, west of downtown. The highest natural point in the city of Miami is in Coconut Grove, near the bay, along the Miami Rock Ridge at 24 feet (7.3 m) above sea level.^[35]

The surface bedrock under the Miami area is called Miami oolite or Miami limestone. This bedrock is covered by a thin layer of soil, and is no more than 50 feet (15 m) thick. Miami limestone formed as the result of the drastic changes in sea level associated with recent glaciations or ice ages. Beginning some 130,000 years ago the Sangamonian Stage raised sea levels to approximately 25 feet (8 m) above the current level. All of southern Florida was covered by a shallow sea. Several parallel lines of reef formed along the edge of the submerged Florida plateau, stretching from the present Miami area to what is now the Dry Tortugas. The area behind this reef line was in effect a large lagoon, and the Miami limestone formed throughout the area from the deposition of oolites and the shells of bryozoans. Starting about 100,000 years ago the Wisconsin glaciation began lowering sea levels, exposing the floor of the lagoon. By 15,000 years ago, the sea level had dropped to 300 to 350 feet (90 to 110 m) below the contemporary level. The sea level rose quickly after that, stabilizing at the current level about 4000 years ago, leaving the mainland of South Florida just above sea level.
Beneath the plain lies the Biscayne Aquifer,^[36] a natural underground source of fresh water that extends from southern Palm Beach County to Florida Bay, with its highest point peaking around the cities of Miami Springs and Hialeah. Most of the Miami metropolitan area obtains its drinking water from this aquifer. As a result of the aquifer, it is not possible to dig more than 15 to 20 ft (5 to 6 m) beneath the city without hitting water, which impedes underground construction, though some underground parking garages exist. For this reason, the mass transit systems in and around Miami are elevated or at-grade.^{[citation needed]}
Most of the western fringes of the city extend into the Everglades, a subtropical marshland located in the southern portion of the U.S. state of Florida. Alligators have ventured into Miami communities and on major highways.
In terms of land area, Miami is one of the smallest major cities in the United States. According to the US Census Bureau, the city encompasses a total area of 55.27 sq mi (143.1 km²). Of that area, 35.67 sq mi (92.4 km²) is land and 19.59 sq mi (50.7 km²) is water. That means Miami comprises over 400,000 people in 35 square miles (91 km²), making it one of the most densely populated cities in the United States, along with New York City, San Francisco, Boston, Chicago, and Philadelphia.