The great Florida Reef, up which our young canoemates had just started on their adventurous cruise, is about 230 miles long. It extends from Cape Florida, on the Atlantic coast, completely around the southern end of the peninsula, and far out into the Gulf of Mexico on the west. The island of Key West lies some 70 miles off the main-land, and about the same distance from the Dry Tortugas, which group of little coral islets forms the western extremity of the reef. Between Key West, on which is a city of the same name containing nearly 20,000 inhabitants, who live farther south than any one else in the United States, and Cape Florida, 150 miles east and north, a multitude of little keys or islands, covered to the water's edge with a dense growth of mangroves and other tropical trees and shrubs, stretch in a continuous line. Between these keys[A] and the main-land lies a vast shallow expanse of water known as the Bay of Florida. Outside of them is the narrow and navigable Hawk Channel, running along their entire length, and bounded on its seaward side by the almost unbroken wall of the outer reef. This rarely rises above the surface, and on it the busy coral insects pursue their ceaseless toil of rock-building. Beyond the reef, between it and the island of Cuba, eighty miles away, pours the mighty flood of the Gulf Stream.