© Pexels / Belle Co
One of the largest known gatherings of sea turtles ever recorded has been captured in an incredible video shared online. The beautiful footage was filmed near Costa Rica as the leathery reptiles were preparing to lay their eggs.

The truly mesmerizing footage was uploaded to YouTube this week by marine biologist Vanessa Bézy; using a drone, she captured the turtle swarm swimming in the Ostional National Wildlife Refuge back in November 2016.

"Everybody I've shown this video has an emotional response," Bézy says. "I immediately knew there was something special going on... To this day, I'm still blown away by the video."

Bézy and colleagues conducted a major study in 2017 based off of their drone footage and associated research to document turtle population numbers and estimated a density of at least 2,086 turtles per square kilometer at the height of the natural phenomenon.

The mostly olive ridley sea turtle (Lepidochelys olivacea) mothers gather offshore primarily between August and October before storming the beaches to begin their synchronized egg-laying. Though numerous in the video, the species is listed as vulnerable by the International Union for Conservation of Nature as their population numbers have been declining.

"It's an enigmatic natural phenomenon," Bézy says. "We don't know how the turtles coordinate this and why."

While the obvious reason for such massive gatherings is to stave off predation, researchers suspect that a number of factors are at play, including optimised weather conditions and increasing the mating opportunities to guarantee the highest yield of new offspring. Though how exactly such incredible numbers of turtles coordinate these gatherings around the world remains a mystery.