For more than two centuries, it was the custom to flood the Piazza Navona during the month of August, generally Rome's hottest month.
The practise started the year after Bernini's Fountain of the Four Rivers (Fontana dei Quattro Fiumi) was completed, in 1651, and lasted until 1867 when the level of the pavement was raised. The creation of what came to be known as the Lago di Piazza Navona took place every weekend in August, when the outlets that carried away the fountain water were stopped and much of the piazza was transformed into a shallow lake (lago).
The coaches of the nobility would then enter the piazza in pairs, one line moving to the right, the other to the left. The windows and balconies surrounding the square would be crowded with spectators, eager to witness the Lago di Piazza Navona.