Montreal Mirabel International Airport — Montréal, Canada

Montreal Mirabel International Airport (YMX) was built in the 1970s as an ambitiously scaled facility meant to become Montreal's primary international gateway, at the time the largest airport in the world by land area. Political, logistical, and location challenges, chiefly its distance from downtown Montreal, meant passenger traffic never materialized as planned, and commercial flights were phased out by 2004 in favor of Trudeau Airport. Today YMX operates as a major cargo, aerospace testing, and industrial aviation hub, hosting Bombardier and other manufacturers who use its long runways for aircraft testing and assembly. Mirabel stands as a well-known case study in infrastructure planning, illustrating the risks of anticipating demand that fails to arrive.