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) and averages at around 6 ft (1.8 m) 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.