Green sea turtles are named not for their shell (which is usually brown or olive) but for the greenish colour of their fat, which comes from the seagrass and algae they eat. Adults can grow up to 1.5 metres long and weigh up to 200 kilograms - about the same as three adult humans. Despite their size, they are graceful and fast in the water, reaching speeds of around 35 kilometres per hour.
Female green sea turtles return to the very same beach where they were born to lay their own eggs - even if that beach is thousands of kilometres away. Scientists call this homing instinct extraordinary. They use the Earth's magnetic field as a compass to navigate across entire ocean basins. On El Salvador's beaches, females come ashore at night, dig a nest in the sand with their back flippers, lay around 100 eggs, cover them up, and return to the sea.
After about two months, the eggs hatch all at once. The tiny hatchlings - about the size of your palm - dig their way to the surface and scramble toward the ocean, guided by the light reflecting off the water. In El Salvador, volunteer rangers and conservation groups patrol the beaches at night to protect nests from predators and help count the hatchlings as they race to the sea.
Green sea turtles are classified as endangered, meaning their numbers have dropped and they need protection. El Salvador has created protected nesting areas along its coast where turtles can lay eggs in safety. Visitors are sometimes invited to watch (from a respectful distance) as hatchlings emerge and make their first journey to the ocean - one of nature's most magical sights.