The Mackinac Bridge is a suspension bridge located in the U.S. state of Michigan, spanning the Mackinac Strait and connecting the lower and upper peninsulas of Michigan.

The bridge serves as the boundary between Lake Michigan and Lake Huron, two of the Great Lakes.

With an initial completion, it ranked as the world's second-longest suspension bridge, surpassed only by the Golden Gate Bridge. Currently, it stands as the fifth-longest suspension bridge globally and holds the record for the largest suspension bridge span in the Western Hemisphere.

The main span of the bridge is 1,158.2 meters, with a road deck width of 14.6 meters. A 0.6-meter-wide central island is incorporated, along with 0.9-meter-wide pedestrian walkways on each side. The bridge was completed in 1957.

The idea for constructing the Mackinac Bridge was first proposed by a Detroit newspaper in February 1884. However, the concept faced debates within the Michigan legislature. The Mackinac area had become a popular tourist destination, especially after the establishment of Mackinac Island State Park in 1875.

In 1888, during a board meeting of the Mackinac Island Grand Hotel, Cornelius Vanderbilt suggested the construction of a bridge similar to those being built in the Firth of Forth in Scotland. The aim was to promote commercial development in the region and extend the resort season for the hotel. Despite decades passing, no formal action was taken, as the area's economy primarily relied on mining and logging, with limited traffic demands.

In 1920, a ferry service was initiated 17 miles east of the bridge's current location in the town of St. Ignace. However, as tourism in the northern Michigan region grew, the ferry service became inadequate. In 1923, the state legislature ordered the State Highway Department to operate ferries across the straits.

As ferry usage increased over the years, and operating costs became high, Governor Fred W. Green in 1928 requested an investigation into the feasibility of building a bridge across the Mackinac Strait.

The department found the idea feasible, estimating the cost to be $30 million (equivalent to $377 million in 2021). In 1934, the Mackinac Bridge Authority was established by the Michigan legislature to explore ways to construct and fund the proposed bridge, with congressional authorization sought for funding.

During the mid-1930s, amidst the Great Depression, many infrastructure projects received federal assistance. Officials twice attempted to secure federal funding for the Mackinac Bridge project, with support from the U.S. Army Corps of Engineers and President Franklin D. Roosevelt, but Congress never appropriated the funds. In 1936-1940, authorities selected the bridge's route based on preliminary studies, and drilling was conducted to study the route's geology in detail.

The initial plan included a three-lane roadway, a railroad passage beneath the bridge, and a central anchorage double-suspension bridge design similar to the San Francisco-Oakland Bay Bridge. However, the practicality of this design was questioned, as it required submerging the anchorage piers in the deepest part of the straits.

Between 1939 and 1941, a concrete causeway approximately 4,000 feet long was built in the shallow water area extending from the north shore. However, the immense forces acting on the bridge foundation posed unique engineering challenges.

The Mackinac Bridge Authority was abolished by the state legislature in 1947, but three years later, in 1950, a new Mackinac Bridge Authority was established. In June 1950, technical personnel were retained for the project. Finally, in the 1950s, construction of the bridge commenced, and in 1957, the Mackinac Bridge was completed, becoming the second-longest suspension bridge in the world at that time.