Most grand avenues are made by widening a street that already exists. The Avenida de Mayo was different: it was driven straight through existing city blocks, a deliberate incision designed by the municipal public works director Juan Antonio Buschiazzo. Begun in 1885 under Mayor Torcuato de Alvear and completed in 1894, it required tearing down part of the historic Cabildo - the colonial town hall where, in 1810, the assembly that launched the May Revolution had met. That revolution gave the avenue its name. To keep the boulevard handsome, the city imposed strict architectural zoning, capping building heights and even commissioning Buschiazzo to design some of the structures himself. The financial Panic of 1890 forced a rollback of the most ornate plans, but the avenue that emerged was unlike anything else in the Americas.

Two-thirds of the way along stands the Palacio Barolo, and it conceals one of the strangest design secrets of any building in the world. Completed in 1923 by the Italian architect Mario Palanti, it was the first structure granted an exception to the avenue's height limits and briefly the tallest building in all of South America. Its proportions are not arbitrary: the whole tower is a model of Dante's Divine Comedy. It rises exactly 100 meters, one meter for each canto of the poem, across 22 floors. The basement and ground floor evoke Hell, the middle stories Purgatory, and the lighthouse-crowned summit Heaven. An immigrant's fortune, built into a vertical reading of medieval verse, standing over a Buenos Aires sidewalk.

At number 825 sits the Café Tortoni, the oldest café in Argentina. Founded in 1858 by a French immigrant and named for a famous café on the Boulevard des Italiens in Paris, it moved to the Avenida de Mayo in 1880 and never left. Beneath its stained-glass ceiling and marble-topped tables, generations of writers, musicians, and politicians have argued and dreamed - the tango singer Carlos Gardel, the writers Jorge Luis Borges and Julio Cortazar among them. To sit there with a coffee and a plate of churros is to share a room with the ghosts of Argentine culture, in a setting that has changed remarkably little in over a century.

The Avenida de Mayo has barely changed since it was built, by design and by law. Its only major alteration came in 1937, when a single block was demolished so the colossal Avenida 9 de Julio could cross it - the two avenues now meet at a junction marked by a statue of Don Quixote, another nod to the Spanish heritage written into the street. Beneath the avenue runs a piece of history too: the first stations of the Buenos Aires subway, opened in 1913, were the first underground railway anywhere outside the United States and Europe. In 1997 the entire avenue was declared a National Historic Site, freezing its façades, marquees, and signs in place. To walk it now is to walk a preserved fragment of the optimistic, European-facing city that Buenos Aires once set out to become.