Secret Cities

With a capacity for 60,000 people, St. Peter's Basilica in Rome is one of the largest architectural designs of its time and one of the most visited monuments in the world.

Secrets of one of the most popular tourist destinations in Paris, the Sacré-Coeur, and its connection to political history and war.

Under the city of London, the underground has continued to develop alongside the city, becoming the world's longest sewer system in the 19th century and eventually home to an organized resistance against Nazis during World War II.