After a Cave Turns Deadly, Scientists Seek Answers

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

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