Californians have been keeping a secret from us easterners. We've heard of the magnificent coastline of Big Sur, of the mountain beauty of Yosemite and Lake Tahoe and of the stark, solitary desert of Death Valley. But what about the other place they go to see all three -- often without the crowds? The place where adventurers can still taste frontier and slake longings for what John Steinbeck called "fantastic and exotic scenery where one finds oneself nodding and saying inwardly, 'Yes, I know.' "?

It is a place right in California's back yard, over the border into Baja, Mexico. A peninsula that stretches languidly to the southeast for 800 miles, Baja is bounded on the west by the Pacific and on the east by the Sea of Cortez, also called the Gulf of California. The two waters meet at the tip of Baja, where huge rocks, one of them chiseled into an arch by the winds and water, jut out from the land.

Baja first drew Indians, who settled in its isolated canyons, then Spanish explorers seeking gold and a sea route eastward, Jesuits in search of earthly fame as cartographers and heavenly reward as savers of souls, and finally prospectors dreaming of gold in the rushes of the late 1800s.

More recently, Baja has exerted its own special lure on Southern Californians seeking the privacy of its deserted beaches, the bounty of its fish-filled waters and the quiet of its stunning desert. And authors often have found it spellbinding. Mystery writer Erle Stanley Gardner came here in the '50s and '60s by jeep and helicopter to explore ancient Indian caves. In l940 John Steinbeck sailed a sardine boat along Baja's eastern coastline with a biologist friend to collect sealife specimens.

It was on this trip that Steinbeck visited the port city of La Paz and first heard a legend he recorded in his log of the trip that began: "An Indian boy by accident found a pearl of great size, an unbelievable pearl . . . " Steinbeck's talent nurtured the legend into "The Pearl."

Today, visitors to Baja can travel in more comfort than ei- ther Steinbeck or Gardner. In a bid to boost Baja's tourism potential, the Mexican government has built a paved highway, Rte. 1, that, with its ancillary roads, offers access the length and breadth of the peninsula.

The route winds more than 1,000 miles, beginning in the northwest corner of Baja -- just south of the border -- that includes the cities of Tijuana, Rosarito and Ensenada. This is a congested, unsightly place. Trucks lugging heaps of fresh fish spew drafts of nose-twisting odor as they rumble to the canneries. Traffic, factories and commercial strips along Rte. 1 whet the appetite for the unspoiled land further to the south.

Once past Ensenada, the road wends through lush northern farmlands and vineyards before offering a choice: Rte. 3 leads to the eastern town of Mexicali and the fishing village of San Felipe, passing through mountains, pine-studded highlands and cattle ranches. Rte. 1 continues south, hugging the Pacific coastline, at times climbing to 2,600 feet and then plunging into alkaline desert. Finally it crosses the peninsula and continues down the Gulf coastline to the resort hotels that have sprung up around the tip of the peninsula -- Cabo San Lucas -- and the town of La Paz, where guests can fly in from California or Tucson to sip a margarita or pin a colada under swaying palms as they watch the sunset's prelude to the stars.

For those who prefer to see Baja by sea, cruise ships regularly ply routes between Los Angeles, San Diego and Mexico, stopping at Baja's Isla Cedros to see elephant seals; at Bahi'a Magdalena to see the great gray whales; and at Cabo San Lucas and Isla Espi'ritu Santo. Ferries also regularly cross the Gulf of California between Baja's eastern coast ports and mainland Mexico.

But Baja is big. And despite the commercial development, those who seek nature in the rough will find it. With a little preparation, a good map, a four-wheel-drive vehicle and camping equipment, the hardy can explore miles and miles of Baja's deserts, with cactus, elephant trees and graceful, swirling cirio trees. And the coastline -- an endless succession of beaches, coves, rocky cliffs and hundreds of uninhabited offshore islands -- is a beachcomber's delight, with crystal clear water for snorkeling, diving, fishing and surfing.

"Welcome. You are in San Pedro Ma'rtir National Park. Beautiful site to preserve wilderness, nature and peace."

My introduction to Baja came this summer in its mountains when I joined a Sierra Club expedition to San Pedro Ma'rtir National Park. It was the first club trip to this 250-square-mile mountain forest park in five years. For eight days our group of 15, all but three of us from California, hiked in elevations ranging from 4,000 to 8,000 feet in a 40-mile circular route. During that time we saw only three other humans: three Mexican cowboys herding their cattle to the alpine meadows for grazing. It was, as the sign at the park entrance promised, a place of wilderness, nature and peace.

The granite ranges of Sierra de Jua'rez and Sierra San Pedro Ma'rtir, which get snowfall in winter, make up the spine of the peninsula. Formed when the San Andreas Fault split Baja from mainland Mexico, these ranges are extensions of the Peninsula Range of Southern California and include the 10,154-foot peak of El Diablo. Reached by a strenuous climb, El Diablo offers views of both the Pacific and the Gulf of California on clear days. (Mexico has built its National Observatory in these mountains to take advantage of the clear atmosphere.)

We carried our own tents and food, cooking on open fires. The hike took us through beautiful woods of pines, cyprus and aspens, canyons carved by dried-up rivers, large, expansive meadows with names like "La Esperanza" and "La Grulla" and hills covered with wild oak, cactus, manzanita, chapparal, wildflowers, mistletoe and the coral-hued "snow plant."

One day included a surrealistic walk through a plateau where huge round boulders lay strewn about like marbles of gods. Some were perched precariously atop granite bluffs, looking as if the slightest breeze would knock them over.

Our most strenuous climb was up Blue Bottle (elevation 9,650 feet), where we had a magnificent view of El Diablo and could see the mountains around us dropping into the far-off desert.

Day-time temperatures rose to about 90 in a bright sun, but a gentle breeze made it perfect hiking weather. At night, it was chilly, often leaving a glaze of thin ice over pots of water. Most of our campsites were by clear running rivers, which sometimes offered small waterfalls and rock pools for swimming. By day the sky was a brilliant blue and at night the stars burst from the sky.

Because the area is not frequented by hikers -- and maps are not always accurate -- the trails are not easy to follow. One day we lost our way for more than six hours. Finally, someone spotted an old cattle trail and for lack of a better idea we followed it, since "the cows always know the way to the river." The trail was choked with vegetation and as she bushwacked through it, one hiker from El Paso, Tex. -- at 77 the oldest in our group -- could be heard mumbling that "no self-respecting cow would ever come this way." The trail led to the river and our campsite.

Our hike ended where it began near the entrance to the park. We then drove down to the foothills of the mountains to the Meling Ranch, one of the few working cattle ranches in Baja. The guest accommodations are on the rustic side -- just the bare necessities for a good night's sleep. But the swimming pool and home-cooked food made the stop worthwhile. Ranch owner Aida Meling Barre, the daughter of Norwegian homesteaders who came to this valley near the turn of the century, serves up the home-cooked food in an informal family style -- and sits at the head of the long picnic table herself. We feasted on refried beans, spaghetti with red clam sauce, garlic bread, legumes, applesauce and green salad. The dining room boasts a huge open fireplace, and on the wall a cuckoo clock marks time.

After the hike, I set out from Tijuana to see more of Baja on my own, specifically the countryside and the southernmost beaches.

From Tijuana, I flew on Aeromexico to La Paz. This sea-side city of white-washed buildings and palm-fringed streets was founded in 1535 by the Spanish explorer Hernando Corte's and gained fame in the 18th century as a center of pearl fishing, an industry that lasted until the 1940s. Today it is the capital of the southern state of Baja: Baja California Sur. (The northern state is Baja California.)

In La Paz, I rented a car and drove along Rte. 9 the 140 miles to Cabo San Lucas. The first half of the 2 1/2-hour drive was through dry scrubland, covered with the tall "giant cardon" cactus. It is the largest cactus in the world, and in Baja it sometimes rises to heights of 50 and 60 feet. In the distance, purple hills rose against a blue sky.

The halfway point was Todos Santos. Set next to the Pacific in a sugarcane-growing region, the village looks like the set for a Mexican cowboy film. Main Street is a sandy road that splays dust as cars pass.

In the heat of midday, no one was at the reception desk of the Hotel Todos Santos. The inner courtyard was empty also, as well as a darkened side room with overstuffed furniture. A dusty-brown notice in the foyer informed that a room cost $10 a night.

From Todos Santos, the drive headed south along the wild and mostly empty Pacific coastline. To the east were the same purple hills. The road passed through a few settlements and farms, but for the most part there was no sign of human life.

San Lucas is a friendly town with roadside taco bars, open-air restaurants and shops selling Mexican artifacts. On a hill in the middle of town is the Church of St. Luke, for whom the town is named.

All around the town, nestled into the rocky cliffs or along the shore, are resort hotels. I chose La Hacienda, a low-slung Spanish colonial-style hotel on the beach. I was greeted with a complimentary margarita and a few choice words from a haughty green parrot that lives in the lobby. The dining room offered a view of the beach and a fare of fresh fish and lobster. At night a mariachi band played in the bar.

From the beach I could see the "Arch" and the "Friars" -- the rocks at the tip of Baja. Legend has it that British pirates sat in wait here for the treasure-laden Spanish galleons returning from the East. On the Pacific side, the strong undertow makes swimming dangerous. But on the gulf side the waters are calm, crystal-clear and safe.

Steinbeck made his expedition to Baja as World War II raged. He recorded in his log how Baja made even that catastrophe seem unreal. "Thousands of miles away the great bombs are falling and the stars are not moved thereby. None of it is important or all of it is . . . Hitler marched into Denmark and into Norway, France had fallen, the Maginot Line was lost -- we didn't know it, but we knew the daily catch of every boat within four hundred miles. It was simply a directional thing; a man has only so much.

"And on the shore the wild doves mourn in the evening and then there comes a pang, some kind of emotional jar, and a longing. And if one followed his whispering impulse he would walk away slowly into the thorny brush following the call of the doves. Trying to remember the Gulf is like trying to re-create a dream. This is by no means a sentimental thing, it has little to do with beauty or even conscious liking. But the Gulf does draw one and we have talked to rich men who own boats, who can go where they will. Regularly they find themselves sucked into the Gulf. And since we have returned, there is always in the backs of our minds the positive drive to go back again. If it were lush and rich, one could understand the pull, but it is fierce and hostile and sullen. The stone mountains pile up to the sky and there is little fresh water. But we know we must go back if we live, and we don't know why."

That is Baja's secret.