Which Italian Gothic cathedral's planned nave was never built due to the Black Death?

Siena Cathedral is the Italian Gothic cathedral whose planned enlargement was halted by the Black Death, leaving its new nave unfinished.

In 1339, Siena launched an immense expansion intended to transform the existing cathedral into the transept of a much larger church. The proposed new structure would have more than doubled the building’s size and helped make Siena’s cathedral one of the largest churches in Europe. A new nave and aisles were planned at right angles to the existing church.

The Black Death reached Siena in 1348 and killed a devastating share of the city’s population. Construction slowed sharply as workers, funds and civic resources disappeared. The project also suffered from serious structural problems, including inadequate foundations and weak building materials.

Work officially ended in 1355 and never resumed. Visitors can still see the surviving walls and the enormous unfinished façade, known as the Facciatone. The present cathedral was not itself destroyed by the plague; rather, it is the abandoned expansion—the “New Cathedral”—that remained incomplete.