Most people come to Tortuguero to see the desove, or egg-laying of the turtles. Few are disappointed, as the majority of tours during laying seasons (March–May & July–Oct) result in sightings of the surreal procession of the reptiles from the sea to make their egg-nests in the sand. While turtles have been known to lay in the daylight (the hatchlings wait under the cover of sand until nightfall to emerge), it is far more common for them to come ashore in the relative safety of night. Nesting can take place turtle-by-turtle – you can watch a single mother come ashore and scramble up the beach just south of the village or, more strikingly, in groups (arribadas) when dozens emerge from the sea at the same time to form a colony, marching up the sands to their chosen spot, safely above the high-tide mark. Each turtle digs a hole in which she lays eighty or more eggs; the collective whirring noise of sand being dug away is extraordinary. Having filled the hole with sand to cover the eggs, the turtles begin their course back to the sea, leaving the eggs to hatch and return to the waves under the cover of darkness. Incubation takes some weeks; when the hatchlings emerge they instinctively follow the light of the moon on the water, scuttling to safety in the ocean.
Tortuguero is one of the best spots in Costa Rica to observe marine turtles nesting, as three of the largest kinds of endangered sea turtles regularly nest here in large numbers. Along with the green (verde) turtle, named for the colour of soup made from its flesh, you might see the hawksbill (carey), with its distinctive hooked beak, and the ridged leatherback (baula), the largest turtle in the world, which weighs around 300kg – though some are as heavy as 500kg and reach 2.4m in length. The green turtles and hawksbills nest mainly from July to October (August is the peak month), while the leatherbacks may come ashore from March to May.