MONVISO — The oldest tunnel ever built through the Alps is set to reopen this week, giving summer tourists a chance to cross the mile-high border between Italy and France—on foot!

The 75-meter Monte Viso Tunnel was originally constructed in 1480 to connect the Marquisate of Saluzzo, now located in Italy's Piedmont region, with the nearby Dauphiné region in France.

The tunnel was designed to increase trade between the two neighbors while bypassing their common enemy of Savoy, which controlled nearby mountain passes and restricted commerce. Dug at a height of 2,880 m (9,449 ft), the tunnel was an engineering feat in its time and took six years to complete.

Workers used iron, fire, boiling water, and vinegar to bore the tunnel, with the costs shared equally by the governments of Saluzzo and Dauphiné. After Saluzzo's annexation to Savoy in 1601 the tunnel lost its strategic importance, and remained closed on and off for centuries.

The first major renovations began in 1907, when the Italian government teamed up with the Italian hiking association to restore access to the tunnel. A joint project two years ago by the Italian region of Piedmont and French authorities extended the tunnel to its original length, repairing erosion that damaged the footpath over the centuries.

Designed with a height of 2-2.5 m to allow the passage of mules carrying goods, the tunnel now only rises to 1.7 m for most of its length, and remains unlit. It stays shut every winter on the French side to block the entry of snow, but reopens in the summer for tourists to cross from Monviso to the French regional park of Queyras on the other side of the tunnel.

Part of a popular regional hiking trail, reaching the Monviso tunnel requires a three-hour hike from the nearest town in Italy, while it's only a two-hour hike from the nearest mountain lodge on the French side. In a time of border walls and Brexit, tourists in the Alps can walk through a Renaissance-era border that remains open almost six centuries later.