Photo: Francesco Piranesi · Public domain · via Wikimedia Commons
Mud Flood · Rome, Italy
Pantheon steps, Rome
The open question
How does more than a metre of ground pile up around one of the most famous buildings in the world?
On the record
The portico once rose seven steps (1.3 m) above the forecourt. Tiber floods and failed drains raised the ground, covering most of the steps, and until the 17th century visitors had to walk down stairs to the portico.

