Possibly the clearest natural water on earth, visibility measured beyond 40m in a limestone chamber lit by a single opening…Possibly the clearest natural water on earth, visibility measured beyond 40m in a limestone chamber lit by a single opening, giving divers the sensation of hanging in air. The Australian Navy used it to calibrate optical torpedo-guidance systems precisely because the water is so clear. On private farmland, used by the Royal Australian Navy from the 1960s for optical testing. Access is by permit through the landowners, which is why it stayed pristine while the region's other sinkholes suffered fatalities in the 1970s that led to Australia's cave-diving certification system. Anchorage: Inland, road access from Robe or Port MacDonnell. Hazards: No site-specific hazards noted, brief normally. Plus