Sitting in the Venetian Lagoon between Venice and Lido, Poveglia Island has been called the most haunted place in Italy—and quite possibly the world. Today, the Italian government keeps the island closed to visitors. But why? The answer begins with a plague.

The Romans first settled Poveglia sometime around the 5th century CE, as people from the mainland fled barbarian invasions. Its population grew steadily over the following centuries, thriving on agriculture and fishing. But in 1380, following a naval battle between Venice and Genoa, the Venetian government relocated the island’s residents to the mainland and converted it into a military outpost. Poveglia sat mostly deserted for the next few hundred years—and that quiet stretch of emptiness was about to become very important.