Underwater cave diving is a risky business.
Environmental anthropologist Kenny Broad and one of his fellow researchers-cum-explorers know this for a fact after experiencing an "underwater avalanche" -- which is when sediment from the walls of the cave collapses in around the divers.
The harrowing experience, which took place in one of the Bahamas' mysterious blue holes, was captured on video. National Geographic uploaded the footage to YouTube last week.
Watch the video, above.
This, for reference, is how small a space they were moving through:
And below is what it looked like when the avalanche surrounded them.
"All of a sudden, it just started raining down on top of me," Broad explains in the video, noting that either the divers' fin kicks or pressure waves from their movements probably caused the sediment to come loose.
Broad and his team have been studying these complex underwater caves since 2008 as part of NatGeo's Blue Holes Project.
The caves, which serve as aquifers and collect the rainwater that filters through the limestone above, are "among the least studied and most threatened habitats on Earth," according to NatGeo. They hold a wealth of valuable scientific information, including unique biological, geological and cultural characteristics.
But studying them involves navigating the treacherous and claustrophobic territory, and then taking samples and snapping photos, all of which takes time -- a limited commodity when you're underwater.
And there's always the risk of unexpected complications, including underwater avalanches, which can result in little to no visibility.
As a precaution, the team always runs a guideline (think Theseus and the Minotaur) that leads back to the mouth of the cave -- a "golden rule" of cave diving.
"You have to assume that you're gonna come out in no visibility," Broad says in the video. "Without that line, you have low odds of getting out."
In this instance, Broad was able to find the line quickly, connect with his dive partner and swim out of the narrow passageway. In wider caves, however, finding the guideline can take much longer.
"It's only when you get to the surface and you see all your dive buddies next to you, you realize just how intense an experience you just went through," he said.
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