The Pantheon started life as a temple to all the gods, built by Marcus Agrippa in 25 BCE. What you see today, though, is mostly Hadrian's doing. That emperor had a habit of rebuilding monuments and leaving the original dedications intact, which confused historians for centuries.

The engineering remains staggering. Ancient Romans invented a volcanic ash concrete that let them cast five thousand tons of it over a wooden frame, creating a dome that's a perfect sphere, 43.3 metres across. The walls are six metres thick. Each of those columns weighs sixty tons, dragged from Egypt on sledges, floated down the Nile, shipped across the Mediterranean, and hauled up from the port.

Since 608 CE, it's been a church dedicated to Saint Mary and the Martyrs, which saved it from being plundered for building materials. Raphael lies here, as do Italy's first kings.

VoiceMap's self-guided audio tours trace how this temple became both a Renaissance inspiration and a source of bronze for Saint Peter's Basilica.