This cistern was approximately 40 feet below the access point. It incorporated two bulb shaped chambers connected by a narrow passageway. Piles of debris were found at the center of each bulb, directly below cistern access points. The water was very clear making for some interesting video footage in the narrow passage. A short video of what the robot saw can be found here.
Shown below is the sonar data image.
California Polytechnic State University
Marine Resources Development Foundation