In the Hadrianic period a huge cistern (opus latericium) was set against the outer back wall of the Foro di Porta Marina (IV,VIII,1). It was connected by a short arch with the city wall, which carried a branch of the aqueduct. In the interior a row of piers supported the vault. Exterior buttresses were built to withstand the pressure of the water. On the east side is a staircase. Later some modifications were made, perhaps in the third century (opus vittatum). The internal measurements are 5.50 x 13 m. The capacity may have been c. 215.000 litres.