The dense jungle surrounds us, pulsating with the sounds of roaring howler monkeys and chattering birds. Clambering after the agile, barefoot guide, my companion Cris and I clutch at the jagged rocks lining the dried riverbed, eager for any leverage up the steep, uneven slope. My hand reaches automatically for a large boulder, covered in gorgeous, emerald-green moss.

"The snakes know you're coming before you ever see them," our guide Esteban warns, and I've obviously forgotten what I was told back at the tent camp. Don't touch before looking--don't assume. An hour later, we trod into the space of a deadly fer-de-lance viper, its brown coils blending perfectly with the river's edge.

Though we're surprised by its presence, it is unfazed by ours, having sensed us long before Esteban spotted its inconspicuous form. We're vain enough to think the snake might attack, but it isn't interested in humans. The snake is the teacher here, and we are learning much about life in the wild.

For a novice naturalist like myself, such moments were welcome education on my nine-day, coast-to-coast exploration of Costa Rica, organized by Costa Rica Expeditions. Featuring everything from tropical beach to mountainous, misty cloud forest, the small Central American country (just 1/8 the size of California) is a haven of natural diversity. About 12% of the total national territory--1,309,000 acres--is protected, unspoiled land. On it thrive 12,000 varieties of plants, 237 species of mammals, 848 kinds of birds and 361 different amphibians and reptiles, many found nowhere else on earth. Costa Rica's 29 park and reserve areas offer visitors an unrivaled glimpse into this exotic world.

Our adventure began in Tortuguero National Park, a mosaic of deltas situated on the country's eastern Caribbean coast. Nearly impassable by vehicle, the 46,500-acre park is most easily reached by small plane or boat, and its remote locale adds to the primitive, "Congo-like" mystique of the region.

Such privacy is an obvious benefit to the animal population, as our three-hour canal boat ride from "mainland" Limon to the park confirmed. Snowy egrets sailed above us as we sped down tranquil canals lined with seemingly impenetrable vegetation. Gangly blue herons perched on fallen branches, eager for the next dawdling fish. The foreboding anhinga bird, spreading its impressive black wings to dry, stood as sentinel to the verdant rainforest beyond. By the time we reached Tortuga Lodge, situated between the jungle and Tortuguero Canal, we were well in tune with the rhythm of the rainforest.

The lodge, operated by Costa Rica Expeditions, harmonizes both the human and animal worlds: elegant, yet simple enough to blend with its primitive surroundings. Subtle two-story buildings (housing four rooms each) face the canal, fronted by porches complete with rocking chairs ideal for lazy, mid-day reflection. The rooms are spacious and open, their wood accents, pale tile floors and ceiling fans a welcome contrast to the often humid Tortuguero climate. Screens are all that protect guests from the backyard jungle and a few vampiric mosquitoes.

The immediacy of the wild is evident. When a troop of howler monkeys parked themselves in the trees behind my room, their territorial bellows sent me through the roof. I was sure Cris was being ravaged by a pack of lions while on his afternoon walk!

A short, circular trail behind the lodge allows for even closer views, though patience and silence are required. Within 15 minutes, we had spotted trail-blazing leaf-cutter ants, fire-engine-red poison-arrow frogs and a seemingly inanimate sloth. The expansive canopy and enormous leaves (called "poor men's umbrellas" by the locals) proved worthy shelter from a violent afternoon rainstorm. The storm seemed the only thing capable of interrupting the sloth's meditation, and you could almost hear it muttering as it crawled for coverage.

Early morning and afternoon boat tours take visitors into the heart of Tortuguero's animated, marshy jungle. Splicing into the thick, ferociously vegetated park, the canals offer an excellent perspective for wildlife viewing, and paired with the excellent "eagle eyes" of the trained naturalist guides, offer many surprising sightings.

What may look like a termite nest to the untrained eye may actually be a ghoulish sloth, languid during its daily 18-hour rest. That brown branch floating in the water might, upon further inspection, peer at you, revealing a sly, crocodilian caiman, waiting patiently for its next winged meal. Other spectacular characters, such as the rowdy spider monkey, brazen keel-billed toucan and stoic iguana, add to the fun.

However exotic the rainforest canals of Tortuguero may be, they are a mere dress rehearsal for rugged Corcovado National Park, located in the Pacific southwest. A gravel runway and rustic pulperia (soda bar) are all that greet the visitor flying into Corcovado's Carate airstrip, but the lack of civilization seems fitting in the "Amazon of Costa Rica."

Situated on the border of the eight habitat, 103,000-acre park, Corcovado Lodge Tent Camp, also owned by Costa Rica Expeditions, is a tropical paradise, and a true escape. Safari-style tents strewn with red hibiscus flowers face a crashing, open Pacific surf, and hammocks swing between tall palms.

The lodge and "hammock house" bar are built on the upward slope of the jungle behind the tents, their thatched-roofs and open walls creating an idyllic scene as scarlet macaws cry overhead. After the amazingly scenic but bumpy plane ride, I was delighted to rest in my comfortable tent and watch the hypnotic ocean before me.

At Corcovado, the hikes are both strenuous and rewarding. The park is rife with activity, representing 10% of the mammals in the Americas.

Our half-day trek down the Rio Madrigal river (during which we encountered our complacent fer-de-lance) was my most memorable in Costa Rica. A native to Corcovado, our guide Esteban proved an encyclopedia of the uses and medicinal qualities of the flora and fauna along the trail. He also challenged us physically, forging through dense foliage and up dramatic slopes, kindly rewarding us with a swim in a pool hidden deep in the jungle.

Neon golden-web spiders and the ever-industrious leaf-cutter ants also traversed the trails, and spider monkeys rustled gregariously in the canopy. During this trek, I was witness to the most awkward rainforest sight of all--Cris swinging clumsily from a dangling root, Tarzan-style. This was his second attempt to commune with nature, having first delivered a howler monkey call so freakish that he sent the entire troop scrambling.

Later, our thirst was quenched from the milk of coconuts fetched by the acrobatic Esteban, and the day (also my birthday) ended superbly with a candlelit dinner and home-baked, banana birthday cake, decorated with hibiscus flowers.

In sharp contrast to the blazing, tropical beaches of Corcovado was Monteverde Cloud Forest Reserve, a 26,000-acre reserve extending down both slopes of the Tilaren Mountains, northwest of San Jose. Misty pastoral scenes and cool forests greet hearty travelers after the two-hour van drive up the rough mountain road, purposely left unpaved to deter an overabundance of tourists (thus protecting the area's delicate ecology).

At the end of the trail, Monteverde Lodge is the most luxurious and sophisticated of the three lodges, and features spectacular views of the sculptured garden and surrounding cloud forest, as well as some excellent birdwatching on its grounds.

As always with Costa Rica Expeditions, both the service and dining at Monteverde Lodge are impeccable. Chefs showcase local specialties, from fried plantains, rice and beans and pico de gallo (hot sauce) in the morning to coconut ice cream as an after-dinner dessert. The staff attends to every detail of your stay, from arranging your transportation around Monteverde to mixing Tico Sours with just the right amount of the local sugarcane spirit, guaro. The cool, darkened hallways and meditative atmosphere imbue guests with a sense of contemplative harmony, and a "rustic trail" on the grounds leads to a scenic, babbling stream, perfect for solitary nature viewing.

The guided walks in Monteverde Cloud Forest Reserve are equally magical, leading guests through dark groves of trees dotted with emerald toucanets, three-wattled bellbirds and (on a lucky day) one or two gorgeous "resplendent quetzals." Other highlights on our own guided walk included a somber tarantula resting in her lair and an aggressive coatamundi (long-nosed member of the raccoon family) in transit. Waddling across our trail, the comic fellow surveyed us carefully with its panda-like eyes, and then plodded into the foliage, its scheming air not unlike its more familiar cousin.

The weather in Monteverde is as diverse as its wildlife. Swift-moving clouds often dump spontaneous rain showers on the area, but the episodes last only a few minutes, quickly restoring the sunny weather. The cloud forest most closely resembles the deciduous forests familiar to us in the United States, but its abundance of rainforest life demands the kind of wet climate that defines the area. I actually enjoyed the volatile weather after the balmy days in Tortuguero and Corcovado, and was happy to wander on trails dusted with dew.

Upon returning from my Costa Rican tour, the melodious sounds of the jungle were replaced by the blare of car horns and the bustle of the urban world. Surprised at my resistance to adapt to my once familiar surroundings, I found myself eagerly drifting back to my contemplative days in the rainforest, where my world was expanded daily.

Luckily, the diversity of Costa Rica makes it impossible to find closure in one visit, encouraging many returns. I can already visualize the majestic anhinga, spreading its wings to greet me back into its mysterious world.