Geoscientists at the Universidade de São Paulo have confirmed the extraordinary scale of the Casa de Pedra cave in São Paulo state's Alto Ribeira region. Using LiDAR sensors carried by drone, researchers from the Institute of Geosciences measured the cave's entrance portal at 197 metres, a figure that may yet grow when the topmost section is included, cementing its status as the largest cave mouth in Brazil. Five Christ the Redeemer statues stacked on top of one another would not reach the top.
The project, run in partnership with the state Forestry Foundation and experienced local speleologists, is about much more than record-keeping. The roughly three-kilometre cave has been closed to visitors since 2003, after a flood accident in which two people drowned, and the team is now instrumenting the entire system to understand exactly how heavy rainfall becomes dangerous water surges underground.
Early monitoring data illustrate the hazard. Rainfall of 60 millimetres per hour raised the water level by 2.17 metres, a 50-millimetre event in December produced a 1.90-metre rise, and the peak water level follows peak rainfall by only about an hour and a half. Quantifying that response is the first step towards a warning system that could one day allow the cave to reopen safely, and towards understanding flood dynamics in karst systems more generally.