I just saw a very interesting local documentary on the underground qanat system beneath the Archbishop's Palsce in Valleyta. It is believed that it has been built by Ottaman slaves for the knights of the Order of St John during the Great Siege.

After some research, I've found that are several qanats in Malta, not only in Valletta but also in Mdina and Rabat.

https://vassallohistory.wordpress.com/the-rabat-qanat/