The full moon, slipping in and out of a thin veil of clouds, made the night sky as pale as buttermilk.

The power launch sped down the narrow canal, lined with dense tropical growth. The sky was reflected in a silvery ribbon, an hourglass shape of reality and reflection meeting at each bend of the waterway.

Within an hour, we would be on an equally moonlit Caribbean beach, watching from a few feet away as a 300-pound green turtle dug a deep nest in the sand and laid her eggs, in a ritual older than humankind.

It was the highlight of a three-day stay at the Rio Colorado Lodge in northeastern Costa Rica. We were in a group of 10 average tourists from three nations who had come to see the region's wildlife at water level, an adventure that has few parallels for anyone interested in the natural world.

Costa Rica is one of the unsung pleasures of Central America. This peaceful democracy, sandwiched between Nicaragua and Panama in the narrow part of the Central American isthmus, is more modern than most of its neighbors. A nation the size of West Virginia, and only slightly more populous, it is a land of high mountains where brightly colored birds lurk in "cloud forests" (rain forests located on mountain ridges of the Pacific Coast), of active volcanoes one can view from the rim and of clean-swept beaches and fresh-water lagoons.

There are only two large cities: San Jose', the capital and usual arrival point; and Limo'n, the major seaport on the Caribbean coast.

There are also along that Caribbean coast a few lodges that cater to sportsmen trolling for snook and tarpon. The Rio Colorado Lodge, open year-round, offers a two-day tour that is a bargain at $195 per person, including overland and waterway travel to the lodge and air-taxi back to San Jose', as well as lodging and meals. For $50 more, in late summer, you can spend another night and watch the turtles come ashore or enjoy the sport fishing that is the principal business of the lodge.

The tour begins in San Jose' in a mountain valley served by several airlines. We were picked up at our hotel at 5:30 a.m. in a comfortable van.

In the clear predawn light we climbed the verdant hillsides to the old capital of Cartago, with its historic cathedral, and then wound down through coffee plantations, bananas and sugar cane, stopping along the way for a massive Costa Rican breakfast of fruit, coffee (of course!), eggs, rice and beans, toast and honey.

Then, with stops whenever we wanted to take pictures, we drove on down to the coastal plain, suddenly reminiscent of Jamaica (and largely populated by a bilingual people whose forebears came from Jamaica).

At 10:30 we boarded a comfortable, diesel-powered, open-sided launch, with plenty of room to move from side to side, for what would be a five-hour trip to the lodge.

There are no roads in this northeast corner of the nation. The coastal rivers, as they meet the Caribbean, have built narrow, thickly wooded sandbars and wide beaches. Behind them is a network of clear, dark lagoons, connected by man-made canals to create an 80-mile water highway from the port of Limo'n to the border with Nicaragua.

It is a comfortable, glass-smooth ride. Our competent bilingual guide sliced up fresh pineapple snacks. There were beers and soft drinks in a cooler, and later we went ashore for lunch at the ranger station at Tortuguero National Park, which we would later revisit to see the turtles. There was enough diversion that there hardly seemed time for a half-hour nap. We slowed and stopped often to photograph the passing scenery.

We saw five kinds of heron; brilliant trogons and toucans; the anhinga (cousin of the cormorant) diving for fish; the huge hanging nests of the bright yellow oropendula; monkeys cavorting in 50-foot trees; three-toed sloths hanging from branches; alligators; coconut palm plantations and fishermen in narrow dugout canoes, or pirogues.

Then, in the late afternoon sunlight, we turned into the Colorado River, 300 muddy yards wide as it nears its sandbar marriage with the sea -- and an inn straight out of Somerset Maugham.

Begun 11 years ago as a fishing lodge by an American, the Rio Colorado Lodge is a cluster of buildings set on stilts against periodic floods and connected by narrow walkways -- with a cacophony of parrots, macaws, monkeys and even a pair of spotted ocelots caged in the open spaces.

We stowed luggage in simple bedrooms with private baths, got a drink in the library-bar and went up on a deck to watch the sun set over a river still busy with fishing boats. Then we had a family-style dinner of fresh-caught snook, pork chops, plaintains and more.

The next morning, before breakfast, we walked through a small village just behind the lodge: simple wooden homes on stilts; fishermen drying nets or repairing tackle; children chasing down wandering hens to collect eggs for breakfast. (Had we wanted, for a few dollars more, we could have rented gear and a boat to go sport fishing at dawn.)

After breakfast, we reboarded the boat and motored across the Colorado and five miles up the last lagoon to the north, the crashing Caribbean surf just a quarter-mile away across a sandspit of palms and undergrowth, then anchored to swim in the clear, fresh water.

After lunch, some of our group flew back to San Jose'. On a clear day, you get a bird's-eye view of two active volcanoes, craggy bowls as neatly defined as the classic Vesuvius, with clouds of steam and scarred flanks from the latest outpourings.

But a few of us had decided to stay an extra night, and after supper went back down the canal to Tortuguero National Park, a wildlife preserve for the endangered green and hawksbill turtles that nest here from early August to early October.

We docked near a village and ranger station and walked out to the wide beach. Almost immediately we saw a turtle in the surf -- a yard long, more than two feet wide, her carapace a dark, glistening bubble in the froth.

Hurrying up the beach to get a good look, we may have frightened her back into the sea; or she may not have been ready to nest. The turtles sometimes venture ashore several times before finding a nesting spot to their liking. Each trip leaves a track like that of a small bulldozer, as powerful flippers churn along like treads, the heavy body smoothing the sand between.

We saw two more -- neither yet ready to nest -- walked up to them, patted their huge shells, examined their front flippers to see that they had been tagged by park rangers when they nested a few years earlier and let them go back to swim a while longer.

At almost 10 o'clock, we saw one that meant business, and carefully kept 30 or 40 feet away as she began scooping out a shallow pit two yards in diameter, the sand flying in the moonlight.

Then, as she settled down to dig the real nest in the moist sand, we were able to move up closer, ultimately to use our flashlights right next to her, and watch a marvel of excavation.

She dug with her hind flippers, exactly as a human might use cupped hands, throwing each flipper full forward and out of the way. Right, left, right, the hole deepening until it was deep enough to hold a volleyball.

The tactile sense of the flipper was extraordinary. With each scoop she felt the edge, excavating a perfect cylinder. As it deepened, she raised her massive body on her front flippers so that the rear flippers could dig deeper, looking a bit like an overweight ballerina rising on point.

Finally she was ready, and we watched again as she began to lay her eggs. In an hour, she would deposit almost 200 eggs, like tough, white ping-pong balls; then she would fill the hole with sand, tamp it down carefully, partially fill the wallow with dry sand and finally return to the sea after a three- or four-hour ritual. She was in a trance so deep that our flash photographs -- and a ranger tagging a front flipper -- did not disturb her.

Forty to 60 days later the turtles would hatch. Those that survived natural predators would be back in perhaps eight years to renew the cycle.

We did not, of course, see them hatch. But it will be worth a return trip one day in late fall to watch that frantic scene as hundreds of inch-long turtles scamper across the beach, braving the seagulls to plunge for the first time into the nurturing, perilous sea.