St. Mark's Square floods due to a phenomenon known as 'aqua alta' or high water. This occurs when high tides combine with certain weather conditions, causing the sea level in the Venetian Lagoon to rise. St. Mark's Square is one of the lowest points in Venice, making it particularly susceptible to flooding. For a more in-depth look at why Venice floods and how the city is adapting to this challenge, check out this detailed article on [Venice flooding](https://compassandpine.com/europe/italy/venice/venice-flooding/).