A deadly mystery lingers in a cave in northern Spain. A sign at the entrance warns visitors not to enter.
For decades, speleologists have trained inside CJ-3, a 164-foot-deep cave in Cañon del Río Lobos Natural Park in the Soria province. But in 2014, visitors to the cave experienced something new at the bottom: they nearly suffocated, and one person fainted. The oxygen levels had suddenly, and inexplicably, dropped.
The unusual incident prompted park officials to contact geologist Rau