After a Cave Turns Deadly, Scientists Seek Answers

By Anna Bitong | January 16, 2017 2:04 pm

Raul Perez Lopez peers into the darkness of CJ-3, a cave that’s mysteriously losing its oxygen. (Credit: Antonio Marcos Nuez)

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 Raul Perez Lopez of the Geological and Mining Institute in Madrid, Spain. Shortly after the first report, daredevil Perez dove into action.

Perez along with two firefighters who specialize in mountain rescues went to investigate CJ-3. In the most dangerous move of his 20-year career, Perez, with the help of a firefighter, lowered himself by rope to the bottom of the deadly cave.

“Upon arriving to the bottom, our gas detector started to beep because there was a lack of oxygen,” said Antonio Marcos Nuez, a geologist and firefighter who entered the cave with Perez to ensure its safety. “I’ve been entering caves for more than 25 years, and such a low oxygen percentage is a very unique situation.”


The oxygen levels in CJ-3 are mysteriously decreasing. (Credit: Antonio Marcos Nuez)

Apart from detectors, they felt the effects of low oxygen: difficulty breathing, exhaustion, nausea and a sharp pain near their hearts. After less than five minutes, they quickly climbed the rope they had installed on the rock and rushed outside. The team did not return to CJ-3 again until December 2016. This time, they went with the proper tools, including breathing equipment that would provide oxygen for an hour.

“(The first time) we didn’t have the right equipment and training and we weren’t prepared for the levels of oxygen that we found,” Perez says. So for two years he’s been learning techniques from firefighters to endure CJ-3’s unforgiving environment.

“Like the astronaut in the movie ‘The Martian,’ I knew the air was going to run out. That was the hardest part for me, the psychological part,” he says. “Without the firefighters, this would have been impossible. We would not have entered the cave and removed the samples.”

His daring commitment to science has yielded some clues about the enigmatic cave. Perez found that oxygen made up just 17.5 percent of the air composition inside the cave, compared to about 19 percent in similarly unventilated caves and 21 percent outside.


Raul Perez Lopez, who is leading the investigation of CJ-3, descends into its depths. (Credit: Antonio Marcos Nuez)

“This cave is exceptional,” Perez says. “There has been a change there in recent years, that’s clear. Even in the deepest caves, the oxygen inside is as good as the oxygen outside.”

To explain the oxygen deficit, Perez tested the cave’s air, measured the amounts of methane and carbon dioxide, and took clay samples from the ground for lab analysis. One early theory: The cave may contain bacteria that’s consuming methane and producing carbon dioxide, displacing the oxygen.

“We know that something is consuming the methane inside and we have to find that source,” Perez says.

He plans to test the air again and take more clay samples in the spring and summer. The new tests would show if oxygen levels change with the seasons. When it’s warmer air blows into the cooler cave. But when it’s colder outside, air rushes back out.

“This is important because the quantity of oxygen that’s introduced in the entrance of the cave can depend on the low or high atmospheric pressure and of the temperature outside,” Perez says, adding that climate change may contribute to reduced oxygen levels. “There may have been a significant change in temperature in the last 30, 40 years that has made the cave blow more air toward the outside.”


Raul Perez Lopez takes measurements at the bottom of CJ-3. He wants to know why oxygen levels are dropping in the cave. (Credit: Antonio Marcos Nuez)

Speleologists have been exploring the cave since at least the 1980s. Could the influx of visitors diminish its oxygen?

“The problem with that theory is that at most, two or three people enter at a time and in the last two years, no one has entered,” Perez said. “And other small caves in this area, also used a lot, don’t have that problem.”

The researchers will perform more tests in CJ-3 in March and also plan to measure the oxygen in nearby caves.

“We have registers that show this process is happening in some caves in the area, but on a smaller scale,” Marcos Nuez says. “A nearby cave, called Las Tainas de Matarrubia, has a very small loss of oxygen, at 19.5 percent. But in no cave has the oxygen gone so low as in CJ-3.”

In the meantime, the park has barred others from entering the cave, which has long served as a speleological tourism destination, says Perez, due to its ancient beauty— it’s also relatively easy to climb (at 164 feet deep, it’s considered shallow compared to other caves with a vertical drop).

Perez hopes to solve its mystery this year.

“We are getting information that I haven’t seen published in any scientific journals,” he said. “This cave has left an impression on all of us.”

CATEGORIZED UNDER: Living World, top posts
  • eric

    This is not a phenomenon unique to this cave. Moaning Cavern in California has the same issue. It is a vertical cave–deep enough to hold the Statue of Liberty, years ago the entire cave was accessible but the lower areas in the last few years are so oxygen depleted that a cigarette lighter will not catch and these parts of the cave have been closed off. I do not remember the reason the guide gave for the cause of the lower oxygen levels– I just remember it being a reasonable explanation.

    • OWilson

      What’s suddenly so mysterious about the air constituents of a confined space?

      We’ve experienced these problems in mines for thousands of years.

      • Rhysem

        The mystery is the change. That’s fairly uncommon. Yes, we have numerous examples of natural and manmade caves/mine shafts with foul air, but they’ve always been foul. Usually, the reason for it is fairly obvious as well, e.g., methane, sulfur dioxide, etc vents. What isn’t very common is for a confined space to suddenly change from 20% O2 to 17.5% O2. That’s a pretty significant shift especially on a non-geological timescale. Although, I am somewhat surprised that it’s “stumping the chefs” so to speak. It’s not like the atmospheric composition of the cave can’t be identified and compared at which point the cause should be fairly glaring, whatever it is.

        • 12 string

          I would start with remote O2 sensors at various depths, continuous air sampling paired with atmospheric changes as well as physically investigating new cracks and sources of air flow at the bottom of the cave. This could be the result of new volcanic processes as well. But in no way should Perez have gone in without breathing apparatus.

          • OWilson

            Once they have it figured out, so what?

            There are millions of caves with millions of soup mixes!

            A way to get a grant for your favorite hobby?

            I’d love to “study” the bleaching of the Great Barrier Reef too.

            Have you seen those yachts? the sundown beach campfire parties, the romance.

            And the hot employees all wear bikinis when they dive!

            Yes, I’m definitely “concerned” about our dying coral reefs, especially in my Canadian winters!

  • OWilson

    It’s a bit of a stretch to blame “Climate Change”. :)

    • Delicia Ambrosino

      To a degree I would agree with you. But even subtle changes in climate can go about changing things in large ways. As in all things, balance is key. What balance is needed inside the cave to promote oxygen? We have become acclimatized to our outer environment but still if one were to take a careful look many things have changed and I don’t just mean the weather. So, even climate change could be a culprit no matter how minute that change may be to some peoples minds. To others the change has been dramatic. I guess you could say I fall into the latter category after observing my surroundings and studying those of far off places for many years now. and then there is the old saying…”Anything is possible”.

      • OWilson

        A butterfly wing, a cough, sneeze or an erupting volcano all contribute in some way to our present circumstance.

        This is beyond doubt!

        But, quantifying the actual effect of each is not at this point, “Settled Science”. :)

    • Buck Sunset

      OWILSON! Don’t be alarmed at climate change. It is changing as we speak as it has been changing for well over 1 million years and it will keep doing so. Nothing remains static except in an absolute vacuum. The False litany is that it is a new phenomena and is man caused. That is the BS part of the story!

  • Aaron Harper

    Without a change in volume, a drop in gas percentage, and thus partial gas pressure indicates an increase in other gas(es). They mention a drop in methane as well, and hint at a rise in CO2, but the key will be in a full gas analysis before and after the change, preferably with regular readings to track the change.

    I would suspect a change in the water or limestone, or bacterial action, probably caused by a weather / climate shift or change in the water table.

  • Uncle Al

    Air average MW = 29. CO2 MW = 44, 1.52 times denser. Take wine glass, mount it upright, add 10% of its volume as wine. How is the liquid distributed? Does it exhibit a “surface?”

    Lakes Kivo and Nyos, same thing. A boundary layer forms given dense input when absent drainage and strong circulation.

  • ConservativismsBest

    The comments here are far more interesting than the article!

  • Ian Brundin MPH RN BSN BSOE

    It may be beneficial to use vacuum filtration looking at microbes, spores, fungi at interval depths creating what is almost a “dark reaction of photosynthesis ” the culprits will thrive on dense O2. The study or examination of this filtrate will have to be done in dense lightless matched environments.

    • Anton Nolan

      Henry. if you consider Edna `s comment is good… on tuesday I got myself them-selves a Jaguar XJ from making $4331 this-past/month or even more money than ten-k this past-month. with-out a doubt this is often the easiest-job I have ever had. I started out this 8-months back and quite straightaway started out to take home a minimum of $81.. per hr. Find the facts…>> FACEBOOK.COM/Tina-E-King-610592265811198/app/208195102528120/

  • okiejoe

    Perhaps(another guess), there had been another connection to the outside which provided ventilation but has been closed by earth movement or water level rise.

  • logical_guru

    The lack of oxygen in the caves is indigenous to that area due to the probable presence of some rocks which must have reacted and consumed the oxygen


Discover's Newsletter

Sign up to get the latest science news delivered weekly right to your inbox!


See More

Collapse bottom bar