Remnants of the Roman era can be found in these excavated water tanks, ancient cisterns that form part of 5km (3 miles) of tunnels under Todi. You can enter just behind Piazza del Popolo. Descending below ground, you walk through a few of the two dozen parallel water-storage cells. There's not a whole lot to see, per se, but engineering enthusiasts will appreciate the ingenious way in which runoff was collected and stored for future use.