Basilica of St.Clement in Rome

You're standing at the Basilica of San Clemente, not far from the Colosseum, dedicated to Pope Clement I. From outside it looks like a quiet church, with a plain façade from 1719 on ancient columns and a small bell tower. But this place is a layer cake of history. Several buildings are stacked on top of each other here, and you can visit the top three. The church you'll walk into dates from around 1100. Beneath it lies a fourth-century church, and below that, Roman buildings from the first century. The story only came to light in the nineteenth century, when the Irish Dominican Joseph Mullooly started digging. Irish Dominicans have looked after the church since 1667.