Willis Tower Skydeck

Willis Tower — which every Chicagoan over 30 still calls Sears Tower — held the title of tallest building in the world for 25 years after its completion in 1973, and at 1,450 feet it remains the tallest building in Chicago and one of the most recognisable skyscrapers on Earth. The Skydeck on the 103rd floor offers views across four states on a clear day, but the real attraction is the Ledge — glass-bottomed boxes that extend four feet out from the building, putting nothing between you and the street 1,353 feet below except a few inches of transparent material.

The building was designed by Skidmore, Owings & Merrill's Bruce Graham and structural engineer Fazlur Rahman Khan, whose 'bundled tube' structural system — essentially nine separate towers of different heights bound together — was a revolutionary engineering concept that made supertall buildings economically feasible. Every tall building built since owes something to Khan's work on this tower.

The Skydeck experience involves a high-speed elevator ride and a queue for the Ledge that varies from 10 minutes to two hours depending on the day. Go early on a weekday morning for the shortest wait, and don't go on a cloudy day — the view from 103 floors up through clouds is atmospheric but you won't see much of the city. On a clear day, the view south toward Indiana, north toward Wisconsin, and east across Lake Michigan to the horizon is one of those perspectives that permanently recalibrates your sense of how big a city can be.