This cistern was approximately 40 feet below the access point. It incorporated a small rectangular chamber with a passageway to a much larger chamber. Another small chamber could also be accessed. All chambers were rounded. This was the deepest cistern (18ft) we encountered, and had 2 access points. This allowed us to drop 2 different robots down, one in each hole, and have one robot videotape the other. Debris was found in 2 different piles. 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