A pool of water in a medieval vaulted chamber underneath the Real Alcazar palace in Seville. It is said to have been made into a swimming pool for the private use of a queen. It is called "Los Baños de Doña María de Padilla"
There are light shafts that come down from the courtyard above into the side aisles adjacent the pool. It causes a very strange light that comes from the sides, like a modern concealed lighting scheme, but it is all natural.