I've always wanted to witness an "arribada" (mass arrival of Ridley turtles) in Costa Rica. The nation is one of only seven countries where the females of this species, uniquely among the world's seven marine turtle species, come ashore en masse to lay eggs.
Until a few years ago, arribadas were known to occur only at Playa Nancite (off-limits to visitors) and Playa Ostional, in Nicoya. More recently they have occurred at Playa Guiones (Nosara) and Playa Camaronal, further south.
During the course of an arribada, tens of thousands of turtles come ashore for three or four nights during July-December (peak season is August and September, starting with the last quarter of the full moon). The mass arrival is a strategy for outwitting predators--or at least of surviving despite them: together they deposit millions of eggs at a time.
Imagine my thrill when I arrived at Playa Camaronal at dusk one day last December to find an arribada in progress.
I watched, awe-struck, as one after another turtle emerged from the surf to drag itself puffing and panting across a wide expanse of beach. Once she settles on a comfortable spot above the high-tide mark, each female scoops out a large body pit with her front flippers. Then her dexterous hind flippers go to work hollowing out a small egg chamber below her tail and into which white, spongy, golf-ball-size spheres fall every few seconds. After shoveling the sand back into place and flinging sand wildly about to hide her precious treasure, she makes her way back to sea.
Now the bad news...
Although most of the important nesting sites in Costa Rica are now protected, turtle populations continue to decline because of illegal harvesting and environmental pressure (all species are now critically endangered). Costa Rica outlawed the taking of turtle eggs nationwide in 1966. Alas, egg poaching is a time-honored tradition. (Coatis, coyotes, raccoons, and other egg-hungry marauders also take a heavy toll on the tasty eggs, too.)
During my visit to Camaronal, I was dismayed to see parents permitting children to climb atop the turtles (!). Adults themselves got far too close to the turtles, including standing in their way as they emerged from the sea. And, most dismaying of all, a young girl offered to sell me some turtle eggs that her mother had scooped out of a nest.
Hundreds of locals had arrived at this remote beach, many quite clearly with a nefarious purpose. Only three rangers (plus one policeman) were present to patrol the two-mile-long beach. And no educational profile was presented upon arrival instructing visitors about how to treat the turtles and their environment with respect.
See my Moon Costa Rica  for complete details on visiting the arribada beaches.