Mesmerising footage shows the greatest density of sea turtles ever recorded

Incredible drone footage has captured video of the largest ever gathering of sea turtles near a wildlife refuge in Costa Rica as they prepare to lay their eggs. Every year hundreds of thousands of female sea turtles arrive at the Ostional Nacional Wildlife Refuge within a few days of each other. Biologist Vanessa Bézy has been studying this phenomenon - called an arribada - and filmed olive ridley sea turtles aggregating in the ocean before reaching shore. She said she saw thousands of turtles per square mile during the filming. Sea turtles are at their most vulnerable to predators when they are tiny hatchlings on the beach and so large groups of mothers lay their eggs at the same time in the same area to increase the chance of their offspring surviving.