Most likely a vertical shaft leading down to join a cuniculus that runs south, beneath wall 7057/7191. Drainage system associated with built tufo drain 7174.
Rectangular cut into bedrock in SE corner of Room 1; Tufo block 'lintel' beneath wall 7057, Western edge of drain;
View from north (1/3)
View from north (2/3)
View from north (3/3)
View from west