The view along Seattle’s waterfront used to be blocked by an elevated motorway known as the Alaskan Way Viaduct. In 2010, construction began to excavate a replacement route in a 52-foot (17.5m) diameter double-decker highway tunnel deep beneath the city. The TBM used to build this project was the largest ever in the world at the time (until the previous entry in our list took its crown) and remains the largest in diameter in North America.

In addition to its size, the project faced another challenge. The machine got stuck early in the mining phase after encountering a steel pipe from a former well, resulting in a major delay to the project, as well as numerous lawsuits.

However, once the machine returned to operation, it completed the 1.7-mile (2.8km) tunnel, and the viaduct has since been removed to revitalise the city’s seafront. The final cost at project completion in 2019 reached $3.3 billion, or $4.5 billion (£3.4bn) today.

20 iconic tunnel megaprojects, ranked by cost