Even the National Park Service's web site for Montezuma Well sinkhole [right, credit NPS] in the Montezuma Castle National Monument lists the depth of water as 55 feet. But an article in the Verde News last week pulls together a fascinating history of the Well's exploration, starting in 1873 and culminating with the 2006 NPS research project that found the real bottom and the source of the water that keeps the 368-foot diameter feature full. Most reports describe a roiling soft slimy silty 'bottom' of the sinkhole at 55 feet.
According to reporter Steve Ayers, the NPS expedition in 2006 found that the supposed bottom is really a layer of fluidized sand in suspension, held in place by the pressure of groundwater coming up from two vents (fractures) in the bedrock. The solid bottom of the Well is at 124 feet depth at the west vent while the east vent is only at 74 feet. The NPS dive team lowered a camera 40 feet into the west vent.