Rome, as it’s often said, wasn’t built in a day. And nowhere is that more evident than its state-of-the-art Metro Line C, an ambitious project meant to help relieve the Italian capital’s renowned traffic hellscape and celebrate its rich archeological history with a unique-in-the-world underground museum.

The €700 million line ($757.7 million) was originally envisioned for the Catholic Jubilee of 2000 as a vital link between Rome’s San Giovanni Cathedral and Vatican City’s St. Peter’s Basilica, making it easier for visiting pilgrims to collect indulgences by walking through the churches’ holy doors.