In the 15th century the exceptional knowledge of hydraulic application is to be found in the architectural manuscripts of the Sienese engineers Mariano di Jacopo and Francesco di Giorgio Martini. They wrote illustrated treatises mainly related to war machines, and also to hydraulic devices, which include, among others, the study of “surprising fountains” and the systems to supply them. The “surprising” fountains were sculptures which were placed in the middle of a courtyard, in a garden, or on a table to amaze people because of the spectacular water movement. This article intends to evaluate the typology of these works of art through the studies done in the 15th century by the Sienese artists in their technical treatises, underlining the connections with the experimental studies on the devices carried out by the Classics.