Shannon Airport — Shannon, Ireland

Shannon Airport, carrying the code SNN, occupies a place in aviation history well beyond its modest size, having opened the world's first duty-free shop back in 1947, a retail concept invented here specifically to compensate for the airport's role as a mandatory refueling stop. Piston and early jet aircraft flying the North Atlantic once had to land at Shannon, on the Clare coast beside the Shannon Estuary, before continuing to New York or Boston, a technical necessity that briefly made this corner of rural Ireland a crossroads for Cold War diplomats, Soviet flights, and transatlantic celebrities alike. Longer-range jets eventually erased that refueling requirement, yet SNN adapted by becoming one of the few airports outside North America to offer U.S. Customs and Border Protection preclearance, letting passengers clear American immigration before departure. Today it serves County Clare and the west of Ireland, including travelers bound for the Cliffs of Moher and the Wild Atlantic Way.