In the course of rescue excavations carried out in an area destined for new construction an articulated system of water drainage was revealed. Located below a narrow plateau, the site is flanked by the deeply cut streams of the Fosso dei Castani to the west and the Fosso Marco Rosso to the east. The structures recovered, cut entirely into the tufo bedrock, consist of a collection and decantation tank, open to the sky, and an underground cuniculus running downhill from the tank, oriented at 150° towards the south-southeast. This structure could have been intended to drain as well as to exploit a water table below the upper plateau. Analogous examples of deviation and channelling of water through underground cuniculi are known from Etruria and Lazio in VI-IV century BC. In the territory of Trevignano Romano, in the immediate neighbourhood of the area under examination, there have been reports in the past of a system of cuniculi and access wells, which could be related to the excavated structures. A capillary system of channels could eventually have been linked to a major conduit such as the Trajanic aqueduct, which might have exploited a previously existing network of cuniculi.