Seven Mile Bridge

Seven Mile Bridge, box girder bridge in Florida that connects the middle keys to the lower keys. It spans from Knights Key, at the western end of the city of Marathon, to Little Duck Key. It was built from 1978 to 1982 and was one of the world’s longest bridges at the time of its completion. The bridge is 6.79 miles (10.93 km) long, and is part of the 113-mile- (182-km-) long Overseas Highway, itself a portion of the 2,369-mile- (3,813-km-) long U.S. Route 1. The Overseas Highway includes a total of 42 bridges that span the Florida Keys.

The Seven Mile Bridge runs parallel to the bridge it replaced in 1982, with the older bridge now commonly referred to as “the Old Seven Mile Bridge” or simply “the Old Seven.” The historic bridge was built between 1905 and 1912 as part of the Florida East Coast (FEC) Railway’s over-sea extension to Key West, also known as the Overseas Railroad. Originally named the Knight’s Key-Pigeon Key-Moser Channel-Pacet Channel Bridge, the construction of the bridge and railroad was financed by railroad magnate and American financier Henry M. Flagler. In 1979 the bridge was added to the National Register of Historic Places.

The Old Seven Mile Bridge

Henry M. Flagler first conceived of the Overseas Railroad when the U.S. government announced its intention to build the Panama Canal; he wanted to build a train link to the port in Key West, the closest deepwater port in the U.S. to the canal. The project was initially dubbed “Flagler’s Folly” because of the difficulties involved in building such a structure. It wasn’t until Flagler found someone who believed in his vision, project engineer James Meredith, that plans for the project were set in motion. It took seven years and more than $50 million for the Overseas Railroad to become a reality.

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The old bridge actually consisted of four different bridge spans: Knights Key at 1.3 miles (2.1 km), Pigeon Key at 1.1 miles (1.8 meters), Moser Channel at 2.6 miles (4.2 km), and Pacet Channel Viaduct at 1.7 miles (2.7 km). The first three were built with reinforced concrete piers and steel girders. The Pacet Channel bridge, technically a viaduct, was built with concrete arches and steel girders. The Moser Channel span incorporated a swing bridge to allow boat traffic to move between the Atlantic Ocean and the Gulf of Mexico.

About 4,000 people, working in two 12-hour shifts to ensure continuous construction, were employed during the building of the railroad. The city of Marathon was named for the project’s difficulties, harsh conditions, and the round-the-clock scheduling of the workers. Five hurricanes, three of them causing major damage, impacted the project during construction, but the bridge was eventually completed on January 22, 1912. Dubbed the “eighth wonder of the world,” the railroad linked the entire eastern coast of Florida and, along with Flagler’s investments in hotels and agriculture, helped establish the region’s tourism and agriculture industries that still thrive today.

The Overseas Railroad operated until the Labor Day hurricane of 1935 destroyed 40 miles of track, cutting off the middle and lower keys from the rest of Florida. The damaged railroad was sold to the state of Florida and Monroe county, which then converted it into a road highway to accommodate vehicle traffic. The tracks were replaced with paved road, and some of the train rails were repurposed into bridge railings. The converted railroad opened in 1938 and became the “Overseas Highway.” It also earned the nickname “Highway of Mirrors” because side mirrors were often knocked off of passing cars on the narrow road.

The modern Seven Mile Bridge

The new Seven Mile bridge was designed by Figg & Muller Engineers and built by Misener Marine Construction, serving as part of a $200 million plan to replace most of the bridges connecting the keys to the Florida mainland. Pier and superstructure segments were precast and manufactured in Tampa and then moved to Marathon via barge. A specially designed overhead gantry lifted each of the superstructure’s 268 spans into place between the pier segments. Construction of the bridge was completed six months ahead of schedule due to the use of the precast segments and an assembly-line-style build. At its highest point, it is 65 feet (20 meters) tall to allow boats to pass below it.

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The project won several design awards, including a Precast/Prestressed Concrete Institute (PCI) Design Award in 1982 and the 1984 Exceptional Award for Cost Saving Innovation from the Federal Highway Administration. The bridge was the longest continuous precast post-tensioned concrete segmental bridge in the world at the time of its completion. In addition to carrying vehicle traffic to and from the keys, a water main is built into the bridge’s box girder body that carries freshwater from the mainland to the keys.

After the new Overseas Highway was built, part of the Old Seven Mile Bridge evolved into a recreation area with a 2.2-mile (3.5-km) walking and biking path between Pigeon Key and Marathon. A trolley was also operated between Marathon and Pigeon Key; in 2006 it was replaced by a ferry. In 2016 it was determined that the old bridge was no longer safe for public use, and a budget was allotted for its renovation. Work on the project, which included structural repairs, new decking, new handrails, and a new ramp to connect Pigeon Key to the bridge, began in 2017. On January 15, 2022, the Old Seven Mile Bridge was reopened for recreation. A new trolley, capable of carrying 60 passengers, started operation the same year.

Both the old and the new Seven Mile Bridge have made appearances in several Hollywood movies, including Mission: Impossible III (2006) and True Lies (1994), with the latter featuring a chase scene that was filmed on a closed-off three-mile section of the new bridge as well as a stretch of the old bridge. Additionally, the James Bond film License to Kill (1989) features an action scene that was filmed on the Old Seven Mile Bridge, and 2 Fast 2 Furious (2003) has a scene with the sunset and the new bridge as a backdrop.

In 2024 Florida state officials announced a plan to completely rebuild the current bridge.