Every month at Costa Rica’s Ostional National Wildlife Refuge, tens to hundreds of thousands of female sea turtles arrive, often within a few days of each other, to lay their eggs on the beach.

Biologist Vanessa Bézy has been studying this remarkable phenomenon, known as a mass arrival, or arribada in Spanish, for years. As part of her research, she filmed the reptiles, mostly olive ridley sea turtles, aggregating in the ocean before reaching shore. One lucky day, she recorded, via a drone, an immense gathering of the creatures—the greatest density of sea turtle species ever recorded.

“I immediately knew there was something special going on,” Bézy says of filming the turtle swarm in November 2016. “To this day