(CNN) — The city of Chicago has long had one of the world's busiest airport systems.

Before the Jet Age and until the late 1950s, Chicago Midway Airport, in the southwest of the city, was the world's busiest airport by passenger traffic.

Its runways, however, were too short for early jet aircraft, and in the span of just a couple of years all flights migrated to O'Hare, in the northwest, which had longer runways and newly built terminals.

The move made O'Hare take over as the world's busiest airport in 1962, a title it held for over 25 years. The high volume of passengers, however, came at a cost: the airport became notoriously congested.

Penalized by the city's unpredictable weather, the runway configuration that attracted jet aircraft didn't stand the test of time.

By the time O'Hare definitively lost the crown of world's busiest airport -- to Atlanta's Hartsfield-Jackson in 1998 -- it was famously overcrowded and losing ground on most performance rankings, often ranking low for customer satisfaction.

The only way forward was a radical modernization of the runway system, which began in 2005 and cost $6 billion. Work was completed in 2021, drastically reducing delays and paving the way for a complete makeover of the rest of the airport -- with an additional $8.5 billion investment -- that will transform it "from curb to gate," the city says (and maybe climb back up those rankings).