The Loggia dei Mercanti, the open-air space beneath Milan’s Palazzo della Ragione, hides a secret communication system. Speak softly to some of its columns, and its archways will whisk your whispers to a well-placed listener on the opposite side of the portico.

The loggia, which is part of a medieval merchant’s square, doubles as a whispering gallery. It’s an unusual acoustic phenomenon that occurs beneath perfectly domed buildings and archways around the world.

The rounded, circular architecture of a whispering gallery allows even the softest sounds to travel to awaiting ears. According to local legends, medieval and renaissance merchants and spies took advantage of the clandestine communication system within the loggia to trade secrets.

Other tales tell of shy lovers or those caught in forbidden romances using the colonnade to whisper sweet nothings to one another. A less exciting version speaks of merchants using the whispering gallery to talk about business above the noise of the busy markets.

Even now, in-the-know tourists can take advantage of the acoustic novelty. Whisper to the internal columns and let the architecture carry your words to someone standing beneath an opposite column. Or, linger along the colonnade and try to eavesdrop on unsuspecting pedestrians across the portico.