US 1 is used to travel the islands of the Florida Keys. Locations along the highway are expressed by Mile Marker numbers. The zero Mile Marker is found in Key West at the Monroe County Courthouse. Mile Marker numbers continue to increase until they pass through the Monroe County line at Mile Marker 112 north of Key Largo. Look for small green signs with white numbers posted at each mile along US1 (these used to be the original mile posts installed along the old railroad bed).

MM 40

As you cross the Seven Mile Bridge from Marathon, Little Duck Key and Missouri Key appear. This is the home of Veterans Memorial Park.

MM 39-38

The next island, Sunshine Key (formerly Ohio Key), hosts a 400-site camping resort with an array of services, making it completely self-sufficient within its own 75 acres.

MM 37-34

Bahia Honda State Recreation Area on Bahia Honda Key has three camping areas with cabins, marinas, and picnic grounds. The water beneath the bridges attracts tarpon and other large gamefish. The Flagler Bridge off the southern peninsula, with the train trestle and the automobile span curving above it, is the original Bahia Honda Bridge, now a national historical site.

MM 33-24

The highway leads travelers into the center of the largest of the Lower Keys, Big Pine Key. The National Key Deer Refuge, located here, is home to about 300 miniature deer, a subspecies of the Virginia white tail deer on the mainland.

Watson's Hammock is a popular nature preserve within the National Key Deer Refuge, and harbors a variety of species. Also located here is the largest body of fresh water in the Keys, known as the "Blue Hole". It is an artificial lake created as a borrow pit, which attracts such freshwater species as alligators, turtles, wading birds and fish.

The Great White Heron Refuge was established in 1938, next to Big Pine Key. It offers protection to rare and endangered species, and is home to many migratory birds nesting here in the winter.

Looe Key National Marine Sanctuary is off-shore from Big Pine, Ramrod and Summerland Keys, all popular centers for divers interested in exploring this underwater site. It was named for the HMS Looe, a British frigate that ran aground in a 1744 storm.

MM 25-24

Summerland Key has its own post office and an airstrip, serving the local residents. There is considerable development on this island.

MM 23-21

Cudjoe Key (MM23), with a larger land mass (3,330.2 acres), is more sparsely settled. In 1960, on the northern side of the Key, a road to the missile tracking station was bulldozed through the hammocks. The station is the home of Fat Albert, the elongated white balloon seen in the sky almost daily.

MM 20-17

Evidence of settlements over the centuries has been found on Sugarloaf Key. In 1910, Englishman C.W. Chase farmed sponges inshore. Other spongers pirated his crop, so he sold his property to Richter C. Perky, who attempted to establish a fishing camp there.

The mosquitoes in the area were so ferocious that Perky built a bat tower in 1929. However, the bats abandoned their post instead of dining on the mosquitoes. Some witty locals claim that the mosquitoes ate the bats! The Perky Bat Tower still stands as a reminder of challenges faced by early settlers.

MM 16-12

The Saddlebunch Keys are uninhabited and covered with red mangroves. The exception is a large residential development on the south side of US1 and a Navy communications site on the north.

MM 11-7

Big Coppitt and Rockland Key are populated with families of servicemen stationed at the Boca Chica Naval Air Base.

MM 6-4

Named for herds of cattle and pigs kept here, Stock Island developed from the overflow of population and commercial ventures. It is now home to Florida Keys Community College and Tennessee Williams Fine Arts Center.