Given that we are currently experiencing a global extinction crisis, the occasion to witness a large congregation of an endangered species is a rare and special opportunity indeed.

So you can imagine how privileged biologist Vanessa Bézy must have felt when she flew her drone over the Costa Rican coastline to capture what might just be the largest swarm of sea turtles ever filmed, reports National Geographic.

The footage, shown in the video above, depicts thousands of olive ridley sea turtles swimming across an area just off Ostional National Wildlife Refuge. It's estimated that there is roughly one turtle for every square meter in the footage. The congregation is so dense that you could almost imagine hopping over the sea by leaping from shell to shell.

In fact, the number of turtles in the video might only be the tip of the iceberg. You can visibly see new turtles rising from the depths as the drone ascends, so there might be many more turtles hidden from view below the surface.

“I immediately knew there was something special going on,” said Bézy, to National Geographic. “To this day I’m still blown away by the video. They look like bumper cars out there.”

Swarms of turtles aren't unusual at Ostional; the refuge was established in 1983 as a protected area specifically for the turtles. But swarms this dense have never been seen before from this vantage point, and they may never be seen again given that this is a vulnerable species.

“This is the only time I’ve seen a video capturing this phenomenon in the water,” explained Roldán Valverde, scientific director of the Sea Turtle Conservancy in Florida. “Most of the photography documenting this occurs on the beach.”

Bézy studies these sea turtles, and says she recently released this footage to raise awareness about what we stand to lose if this species is not protected. Though these turtles are widespread, they have very few nesting sites around the world, and so when these nesting sites become compromised it threatens whole populations at once. For instance, Bézy has become increasingly concerned about a rising tourism industry around the beaches where this population nests.

New regulations have been proposed to protect these critical beaches, but as developers encroach and access becomes easier, the regulations may not be enough. Part of the problem is that olive ridley sea turtle hatchlings have a very low survival rate into adulthood. Only about 1 out of 100 reach maturity, and that's just considering natural threats. If you pile on threats from human encroachment, it's easy to see how the turtle population could crash fast, potentially in a single generation.

For now, as we can see in the video, this population seems robust. Bézy hopes that her footage will help to keep it that way.

“Everybody I’ve shown this video has an emotional response,” she said.

Bryan Nelson ( @@brynelson ) writes about everything from environmental problems here on Earth to big questions in space.

Drone captures the largest swarm of sea turtles ever filmed
The density of turtles is so high that you could almost imagine crossing the sea by hopping from shell to shell.