South of Mazatla'n, perhaps 40 or 50 minutes, everything starts to change. The very air you breathe turns languid, heavy. Herons, cranes and egrets appear in the ditches and fields alongside Mexican Highway 15. The coastal desert is still there, with its anguished rock and tormented brush, but toward the Pacific, beyond the dessicated hills, a new landscape is emerging, of mangrove swamp, winding estuaries, extravagant jungle. You have crossed the Tropic of Cancer, that invisible boundary 23 1/2 degrees north of the Equator that separates the lands of frost and fire, the temperate zone from the tropics. Welcome to Endless Summer, with its rain forests, orchids and warm blue surf.

One of the best places to go in this pleasant zone is San Blas, south of Mazatla'n, north of Tepic, a few miles west of Highway 15 on the Pacific coast. A single caveat: Don't try camping there during the spring, the nocturnal insect season. A friend and I tried, the first time we went there, and were nearly devoured in our sleeping bags by Luftwaffes of venomous sandflies and Brobdingnagian mosquitoes. Two burly tourists from Mexico City bivouacked on a worse section of beach and ended up in the hospital in anaphylactic shock, covered with lumps the size of ping-pong balls.

Night-flying bugs aside, San Blas is lovely. Even the approach, from inland: You descend through emerald fields, forests, past thatched huts, laughing machete-wielding farmers, women pounding laundry in shallow rivers. The Pacific rises, gray and silver, beyond the palm trees. A last stretch of lush tidewater, and you are there.

Life in San Blas is lazy, timeless. The cafe's serve giant shrimp, fried potatoes, leathery steaks and a local sea fish, a kind of mini-barracuda, roasted on the coals: the same menu over and over again, everywhere in town, but somehow you never tire of it. On that first visit and later ones, we drank tequila and Pacifico Beer -- "Pacifico, y Nada Mas!" the radios screamed -- till past midnight at a piratical bar with a concrete tank of sleepy alligators in the center of the room. The regular clientele included a California yachtsman, tan and mustachioed, who had sailed down from Monterey, sunk his craft on the rocks at the entrance to San Blas harbor, and stayed on, beached, stranded, totally bemused; the gorgeous raging poetess from New York, who danced in the 'gator pen, tequila bottle in hand, shouting "Do not go gentle into the good night," until the bartender threw her out; Mel the Cosmic Surfer, with his tales of Great White Sharks, Monster Manta Rays and Thirty-Foot Typhoon Waves ... a rare crew, indeed.

Days, we rode the 30-centavo bus down to Matanchin Bay, through swamp and scrub jungle. The bus stopped right at the edge of the most beautiful beach in the world. Great glassy waves rolled in from far out to sea: According to the surfers, you could ride them three miles before they broke, rides so long that you had to kneel on your board midway to shore, your legs were so exhausted. South, behind the beach, steep jungled mountains rose into the clouds. The scene always reminded me of Gauguin's Polynesia: the same sense of fecund mystery, of a sensuousness at once sweet and strangely forbidding.

Indians came down out of those mountains sometimes, Huicholes, from high up in the Sierra Madre Occidental. According to the stories, young Huicholes went through an initiation in which they were led from their homeland deep in the mountains, blindfolded, to a ridge overlooking the Pacific. At dawn, the blindfold was removed, and the initiate looked out over the infinite sea. "Your soul dwells there," he or she was told. "You are a part of that brilliant, tranquil void, and it is part of you." Whether the initiation story was completely accurate or not, the Indians definitely regarded the sea as sacred. We ran into little groups of them at dawn on the beach: Led by shamans in magnificent feather-decked sombreros, they waded out into the surf and let the turquoise waters wash over them, gazing out toward the wild horizons.

When we tired of the beach, there was always "The Jungle Boat Ride": For a few dollars, one of the local fishermen would take us up the river from town in a leaky motor-driven canoe, deep into the forested marshes. The narrow waterways wound through gloomy tunnels of shadows, pools of fierce sunlight. Iguanas the size of dachshunds, in radioactive colors, clung to overhanging branches. Great blue herons stood and stared. Flocks of parrots soared.

Midway through the ride we arrived at a fairyland cantina, on a ledge above a deep crystal pool. The boat tied up there long enough for us to eat a few tacos, drink a beer or two and swim. People claimed to have spotted alligators, small ones, in the pool, but the stories were always third person, vague; I didn't really believe them, but they gave the swims a nice edge, a feeling that one was playing on the edge of relative danger.

San Blas isn't the only place, of course. There are other, wilder villages further down the coast, accessible only by nonscheduled hit-or-miss bus, or hired boat. No hotels: You stay in rented palapas with palm-frond roofs and sand floors, like "Robinson Crusoe," and live on bartered fish and fruit. Inland is more marvelous territory: the mountain town of Tepic, and villages like Santiago Ixcuintla, Ahuacatla'n and haunted Ixtla'n. They sell tiny, suffocatingly sweet bananas there, the peels red and shiny as lipstick; biting into one is like nipping an ampul of pure banana oil. There are carnivals on saints' days, Easter and Christmas, with flimsy Ferris wheels, sword-swallowers and soothsayers, Christmas tree lights everywhere and blinding, deafening fireworks.

It is definitely a different world down there, south of the Tropic. I recall another trip, in late winter. I took the train from Nogales to Tepic, a delightful, rattling ride, and then caught a local bus to San Blas. It was midsummer: warm days verging on hot, balmy nights, lukewarm seas. The markets were full of pineapples, mangos, papayas. I swam, bodysurfed, rode a rented bicycle through the cobbled streets.

By the time I caught a ride north with a friend, two or three weeks later, I had completely lost track of what time of year it really was. Our route took us north to Mazatla'n, then east, across the Sierra Madres into the Great Mexican Desert. Just north of the city of Chihuahua, driving in the middle of the night, still 220 miles south of the Rio Grande, the air turned cold. Strange luminous confetti began to blow across the headlight beams: We thought it was dust, some kind of impossibly coarse Chihuahuan loess, but no, it was snow -- the southern edge of a huge blizzard, deep in the desert of Mexico. It kept on snowing all the way to the border, to Ciudad Jua'rez/El Paso, and then it snowed even harder. By the time we got to Las Cruces, N.M., the Interstate was deep in frozen drifts. Dawn rose on white hills, blank fields, ghostly ice-covered trees. The warm coast we had left the day before already seemed like an impossible fantasy, a distant memory.

Tropism has an irresistible way of becoming a habit: You keep turning toward the light, the heat, again and again. I haven't been to Mexico beyond the Tropic of Cancer in several years now, but I definitely plan on returning, soon. I have this wonderful romantic vision. I will pack up my portable typewriter, and 600 sheets of erasable bond, and a thin sheaf of traveler's checks, and I will head south when the weather starts to turn cold. I will drive down through the Navajo Nation, across the deserts of southern Arizona, with their towering saguaro cactuses ... down through Nogales, Hermosillo, Guaymas, Mazatla'n. I won't stop until I have crossed the Tropic and arrived at the rim of the Pacific.

Some place down there -- maybe San Blas, maybe Zihuatanejo, or one of those unknown, unvisited villages in between -- I will rent a room behind an adobe cantina, or a palapa under the palms. I'll set up a hammock, and a table and chair, and I'll hang one of those wonderfully gaudy Mexican calendars on the wall, the classic one that shows the shepherd boy on the jungle mountainside, defending his flocks against a giant condor.

I'll live on cafe au lait, mangos and fresh fish; I'll bodysurf every morning from 7 to 9, afternoons from 4 p.m. till sunset. In the evening I'll trade stories at the local alligator bar, till closing.

The rest of the time, I'll be busy writing the Great Meso-American Novel, or maybe the Ultimate Screenplay. I will wake at 4 in the morning, mind full of burning ideas, and I'll sit down at my little table and type them onto paper, page after brilliant page, while the radio murmurs after-hours New Orleans jazz, and the Pacific waves break on the beach outside. The rest of the wintry world will be a thousand, million miles away.

Rob Schultheis is covering the war in Afghanistan for Time magazine.