We'd made it!

The clouds, swirling like wraiths throughout our long climb up the 5,000-foot-high vertical mountain, suddenly lifted. My two Indian guides hauled me the last few feet through the mud and mossy slime onto a slab of cold black rock.

We were utterly exhausted. The ascent had taken two long days, up from the gloom and tangle of the jungle, along slippery ledges hardly wide enough for a toehold, shimmering up wet clefts, and always cursing the constant mists that cut visibility to a few murky feet. Many times I'd thought we'd reached the summit, only to peer through fleeting holes in the clouds and see the rock face rising up, endlessly.

I had begun to fear we'd never arrive. It was hard to tell what my guides thought. Most of their peers kept well away from these strange, pillarlike mountains -- the mysterious tepuis of Venezuela's La Gran Sabana.

But this truly was the summit. As the clouds melted, I could see a barren plateau of incised rocks stretching away into the sun. A cold wind tore across the wilderness, screaming and howling in the clefts. There were no trees. Spongy clumps of dripping moss clung to the more sheltered sides of broken strata. The rest of the summit was as arid as a stone desert of scattered black stumps, worn into shapes like petrified figures.

I was shivering, dazed with fatigue, amazed by the bleakness of the landscape -- and elated. After years of dreams and half-baked schemes, I was finally here -- one of few men ever to stand on this towering tepui in one of the remotest regions on earth, a true "lost world," where you could see forever across an infinity of Amazonian jungle.

I blame it all on Sir Arthur Conan Doyle. In 1912 the creator of Sherlock Holmes released a novel, "The Lost World," that was based on a tantalizing premise: Somewhere on the northern edge of the Amazon Basin, an eccentric English professor had discovered a primeval "lost world" of flat-topped, sheer-sided mountains, thousands of feet high, soaring out of impenetrable jungle. The explorer returned home, after numerous misadventures, to proclaim the existence of a land where time had stood still, a bastion of ancient life forms long considered extinct.

I read the book as a child and vowed that one day I'd find this place. Years passed, but the idea never faded. I learned that Sir Arthur had indeed traveled in southern Venezuela and that these strange mountains, hundreds of 8,000-foot-high tepuis, did, in fact, exist on the fringes of the Amazonian basin.

Finally, I went to Venezuela to see them for myself.

Set high above the steamy coastal plain, Caracas overflows the edges of its mountain bowl, bathed in balmy springlike breezes. The temperature rarely exceeds 80 degrees here. The capital of Venezuela is an invigorating place, teeming with traffic, booming (until recently, at least) with new oil-cash affluence. Scores of high white towers -- real fat-cat architecture -- rise above boulevards and formal parks.

But I hadn't come for city life, no matter how seductive. My only concern was the tepuis of the far southeast. I was impatient to reach my "lost world."

You could hear the excitement in the pilot's voice over the intercom: "The cloud cover is lifting. We should be able to see the mountains. I shall be flying as close as possible to the Angel Falls ... . It will be bumpy."

That was nothing new. The whole journey had been bumpy. Five hundred miles from Caracas, our 50-seat plane bounced across the thermals thrown up by the dry red plains and brittle ridges below us. Then we were up through the clouds, peering out of tiny windows, seeking signs of the tepuis.

"Angel Falls approaching on the right."

And there it was. Tumbling in lacy sprays off the summit of Auyan Tepuy, an uninterrupted drop of 3,212 feet, the tallest waterfall in the world billowed, sheened by the afternoon sun.

There were more falls. Smaller but no less impressive, they spumed off the black cracked top of the tepui and disappeared in a hundred streams far below. Beyond, the hazy silhouettes of other tepuis stretched out across the green infinity into the 1,500-mile-wide Amazonian basin.

After years of dreaming, I had arrived at the edge of Sir Arthur's Lost World.

Base camp at Canaima was a cluster of chalets in the jungle. For most visitors, this was the beginning and end of their journey -- a lovely interlude of meals and cocktails on a shady terrace overlooking the falls, maybe a river excursion to the base of Angel Falls, then back to Caracas and the coastal resorts.

For me, it was just the beginning. After three days of negotiation with the local guides, I set out for tepui country with Tin and Pan, two Pemon Indians.

The river was placid, oily-surfaced. The ripples made by our wooden dugout canoe, which was powered by a modest outboard motor, hardly rippled at all. The sun hammered its surface into submission.

I trailed my hand in the river -- warm as a hot tub -- and then removed it, remembering all those tales of subsurface creatures awaiting the unsuspecting novice -- the giant caiman alligator, whose bite will snap off an arm fast as a die-cutter; the anaconda, a huge river-dwelling boa said to reach 50 feet in length; the piranhas, with a hundred teeth of honed glass set in bulldog jaws.

The jungle eased by, a solid exuberant mass of green, edged in parasol-topped palms. Taken in small sections, it was a senseless tangle of vines, dead limbs, fallen trees, fresh perky foliage, masses of dun-colored leaves, ferns and palm fronds sinking back into the pulpy floor. Taken in larger sections, you could see the calm, changeless form and structure of the jungle, the striated tiers defined by the varied species of trees, ferns and bushes, peaking in 100-foot-high treetops.

We spent two long days of relatively calm riding, and then the peat-brown river suddenly narrowed to swirling rapids; our tiny canoe bounced like a bottle on the frothy current. The jungle closed in, tangled foliage on either bank almost meeting overhead. It was a forbidding stretch of angry water; I felt we were unwanted, like so much useless flotsam, enveloped in shadows, waiting for something unpleasant to happen -- something to pounce, some disaster that would sweep us all away, unnoticed, irrelevant in the great scale of the place.

Rocks rose up, black and pyramidal, topped with green mosses and forest slime. But Pan had eyes for nothing but the water, watching every eddy, avoiding the approaching calm sections that slowly whirlpooled away into the shadows.

The canoe was taking in water. Sometimes there were only a couple of inches between the sides of the craft and the snarling torrent. Tin scooped out the water with his hands. I helped, but it didn't seem to make a difference. The noise was deafening, ferocious.

I watched Pan's mouth. He was either chanting or talking to the river. His body was still, except for that outstretched hand on the rudder. He and the water were one -- not exactly on friendly terms, but certainly role-matched. My fears turned to trusting admiration as I watched his eyes. I knew we would make it.

And we did. Half an hour or so later we were through the rapids and into a wider, calmer stream. The jungle pulled back and sparkled in the afternoon heat. The first trial was over and we eased toward a small rocky beach. Both Indians were smiling. I took Pan's hand and held it. I could have hugged him.

"Thank you," I said.

He squeezed my fingers and grinned a rare grin before leaping out and pulling the canoe onto the sand.

We'd chosen a lousy place to land. Not only was the bank collapsing and root-riven, but if you put one foot wrong you were up to your thighs in putrid black goo that stuck like molasses.

A few minutes later, though, I wished I was covered head-to-toe with the stuff. At least it would have been some protection against the unbelievable onslaught of mosquitoes and a million other blood-sucking bombardiers. For every dozen I smashed, two dozen more took their place.

In spite of all the animal clamor of the jungle, you don't actually get to see much. You know there are monkeys out here -- their howling and screeching follows you as you climb higher through the sticky brush -- but all I caught were shadowy glimpses and shaking branches where they had been seconds before. Parrots are less shy -- but you expect exhibitionism from parrots.

One morning, after a particularly sweaty night, Tin pointed to a set of paw marks near our camp and said quietly, "Tigre."

"Tiger! You've got to be joking!"

But apparently "tigre" doesn't mean tiger. It's usually applied to the jaguar, black creature of the dark shadows, and other smaller members of the cat family that run free through the Venezuelan rain forests.

And then there were the butterflies. What a paradise for the serious collector. Some big as bats, others with harp-shaped wings, some mere shards of electric shimmer -- scarlet, indigo, gold, silver-edged, vermilion -- swirling like colored snowflakes. In moments like these, the ever-present mosquitoes were forgotten. Even Tin and Pan seemed moved as we passed pools where butterflies fluttered by the hundreds over the water and the damp pebbles.

On the third day we awoke to a world of gray. Utterly seamless slate gray -- the sky, the jungle, us. Even the riotous cacophony of howls, screeches and whistles seemed subdued in the dun dawn. There was none of the normal wet heat of morning. I ate a breakfast of roasted plantains with hardly a bubble of sweat on my face, a most unusual and welcome occurrence.

Tin and Pan, however, huddled together like the Indians of the high Andes, hammocks and shirts pulled over their shoulders, sitting close to the fire. For the first time since we'd left Canaima my body felt comfortable.

Of course, it didn't last for long. By the time we'd packed up camp, the sun had broken through in patches and the heat rose by the minute.

It was hard going through the jungle. My guides were way ahead of me, as usual. Then Pan turned suddenly with a raised hand and beckoned. We peered together through the sticky gloom at the path ahead (path, of course, being a euphemism for the almost invisible indentations in the rampant foliage). At first I saw nothing unusual, but, as my eyes focused, the whole jungle floor became a mass of movement, like a rippling green pond. Tens of thousands -- who knows, maybe millions -- of leaf fragments with neatly clipped edges were moving upright through the low ferns, each one carried by a tiny brown ant.

"The ants who carry the leaves," Tin explained.

We watched the procession, like a New York harbor regatta of little green spinnakers, tracking in unison. The ants were oblivious to our intrusion. Once again, I had that sensation of absolute order-in-chaos -- finite patterns in the green infinity.

As we approached our isolated tepui, the view became more elusive. Occasionally the mountain would rear up like a vast totem over the trees. But most times it was hidden behind jungle curtains or thick cloud cover with only its dark base exposed.

While climbing the lower flanks, we lost sight of it altogether. The jungle closed in around us. Tin and Pan improvised a trail through the scratching palm scrub and dangling vines. We were ascending, so at least we were headed in the right direction. But the tepui gave us no clues. Having beckoned us from afar, it now ignored us as we flailed around its muddy slopes.

Then we had our first real moment of contact. Tin was leading us up along a stream bed full of tiny waterfalls and still black pools. It was a wet, tiring slog with no rhythm to it at all -- slime-coated rocks, unreliable handholds and a couple of dousings. Then the jungle drew back, light tumbled into a clearing and a filigree of waterfalls, like floating gossamer, rained down on us.

We looked up and there it was, all 3,000 feet of it, rising straight into a clear evening sky. There seemed to be no way up. The walls were too sheer.

It was a beautiful sight -- and a depressing one. My legs decided they'd had enough and buckled. I just wanted to sleep.

Then the rock face vanished again and the dainty waterfalls floated down out of thick clouds. Maybe it was better that we couldn't see the impossibility of the climb; otherwise I might have given up and set off back to the camp.

There were occasions on that climb when I thought I'd have to admit ignominious defeat. You've never experienced the essence of helplessness until you begin sliding slowly, horribly slowly, downhill on a slime-coated rock with no handholds around and no way of stopping yourself, toward the edge of a 1,000-foot drop into total oblivion.

It was the slowness of it all that still sends tingles to my toes -- that, and what seemed to be the absolute predictability of my fate. I'm still not sure how I escaped, except that by rolling slightly to my left I found a pocket of rock uncoated by slime, and used that as a brake. By the time my slide had stopped, I could feel the updraft of air on the vertical side of the tepui rushing past my face.

I decided not to dwell on the possibilities. If my confidence went, then the whole expedition would have been a wasted effort. So I concentrated on the climb -- one more cleft, a narrow ledge swirled in mist, then an easier stretch up a 45-degree incline, then a cleft again. On and on and on.

The summit was all I had hoped for and more -- a barren black wilderness of worn rocks, as remote and bleak a landscape as you can imagine. Another world. Another planet.

We floated in limbo. Above, nothing but blue in all directions; below, a cotton-ball landscape of low clouds, cutting out views of the jungle. And rising like fantasy castles in the distance, the great tepuis themselves -- enormous islands in the sky, all flat-topped, each one unique in bulk and profile, ancient keeps of ancient life forms, untouched and unexplored since the beginning of time. I thanked Sir Arthur Conan Doyle again for bringing me here.

According to my map, the summit of our tepui was roughly circular and only a couple of miles across from one rim to the other. I'd hoped to spend a couple of days up here, but hadn't realized how cold it would be. In spite of the open skies and brilliant midday sun, it was hard to keep warm in the biting wind that screeched across the rocks.

Tin and Pan showed little interest in exploring. They found a small hollow behind one of the eroded pillars and, using their hammocks as blankets, huddled together, looking frozen and forlorn.

I was too excited to sit and told them I'd be back shortly after a little exploring. They nodded glumly and huddled closer together.

Based on cursory studies, botanists estimate that there may be as many as 4,000 plant forms unique to these summits. Conan Doyle was right in principle. The isolation of the tepuis has given us a living laboratory of hitherto unknown species.

At first I saw little signs of life. The clefts between the eroded pillars were damp, puddled and bare. At one point, I had to squeeze sideways to pass between the monoliths, only to find myself peering straight down a deep fissure that seemed to have no bottom. A small stream trickled off a ledge and disappeared.

Here I found a rich grouping of lichens, mini-succulents and a cluster of brilliant red flowers, only an inch or so across, with yellow-edged petals. Miniature orchids, tiny purple and lemon extravaganzas, were nurtured by dark green moss. Fifty yards on from the murky pit, I almost crushed a group of green and pink flycatchers, only a few inches high.

Moving on deeper into the labyrinth, I lost track of time. Carelessly, I'd left my watch back in my rucksack with the guides. Now I realized it had become much darker. The blue sky had been replaced by clouds -- not the happy puffball variety but that cloying mist again.

I'd mentally registered a series of distinct rocky landmarks into the labyrinth, but suddenly it all seemed very different. I decided to go back to the starting point on the rim and see how bad the cloud cover really was.

I found the flycatchers, but couldn't see far enough ahead to the next reference point. Somewhere in the gloom was a protrusion shaped like an ape's head. At least that's what it looked like from the other side, but from here ...

One cleft on my right seemed familiar, so I edged my way between the rock walls for a hundred feet until it ended abruptly. There were enough handholds to scale the wall. Maybe I'd be able to see my starting point from the top.

Covered in moss and mud, I eased myself up into a world of whirling mist and nothing else. I retreated into the cleft and back to the reassuring clump of flycatchers. Only they weren't my flycatchers. There were only three of them and they were the wrong color.

Okay. Hold on. No reason to panic. Just a slight error of orientation. If only I could find the ape's head. I couldn't be more than a hundred yards from the rim.

Then came the thunder. It began as a gentle rumble way off among the tepuis, then headed straight for the labyrinth with ground-cracking fury and climaxed in a shattering roar right over my head.

Time to panic.

I matched the thunder blow for blow. Between the booms I roared out the names of my guides into the mist. (Tin and Pan. How ridiculous can you get?) Nothing. Another boom. Tin! Boom! More nothing.

I was angry with myself, with my guides, with the thunder, with this stupid maze of rocks. I was so angry that I almost stepped into the fissure with the little disappearing stream.

What a wonderful fissure! What a lovely little stream! Now at least I knew where I was.

The guides were just where I'd left them, fast asleep, oblivious to the din and the cold mist swirling about them.

I decided not to tell them about my lousy sense of direction and fumbled instead for my watch. It was 2 p.m. We had five, maybe six, hours of light left. I wanted to stay on the summit, but felt decidedly unwelcome now.

"Can we get back down to the trees in five hours?"

Two sleepy heads nodded enthusiastically. Their hammocks were rolled in seconds, and they were ready.

One last look. The mist was still swirling around the black rocks. So much to learn up here. So much to discover. These islands in the sky don't relinquish their secrets easily. The careless will be punished, and I'd been too careless by half already. I prayed for a safe descent and an uneventful return to the comforts of Canaima Camp.

A soft bed, a decent meal and a few cold beers suddenly seemed very appealing.

David Yeadon is author and illustrator of many travel books, including "The Back of Beyond -- Travels to the Wild Places of the Earth" (to be published this year by HarperCollins). He is currently at work on "Lost Worlds -- Exploring the Earth's Remotest Places" for HarperCollins. WAYS & MEANS

GETTING THERE: Venezuela is still one of the travel bargains of the world. Round-trip air fare to Caracas from Washington via American and Pan Am can run as low as $339, with restrictions. Venezuela's Viasa International Airways offers cut-rate packages to Caracas that include air fare and hotel. For example, a four-day, three-night package from Washington to Caracas with accommodations at the Anauco Hilton currently costs $549 round trip.

CANAIMA TOURS: There are many reliable tour operators in Caracas. Avensa Airlines offers a three-day, two-night excursion from Caracas to Canaima at $457 per person ($550 per couple), including round-trip air fare from Caracas, lodging, meals and a boat ride.

Many travelers are content to spend a couple of days in Canaima, with side trips to Yuri Falls, Orquidea Island or Ratoncito Island. More adventurous visitors can take "Jungle Rudy" Truffino's (Box 61879, Caracas 1060, Venezuela) four-day river trips (usually offered from July to November) to Angel Falls for about $300 per person -- or they can make arrangements at Canaima for a wider range of exploration options. Lost World Adventures in Georgia (1-800-999-0558) also offers many different tours of the region. One example: a 12-day, 11-night excursion from Caracas to Mount Roraima, one of the flat-topped tepuis, including meals, guides and hiking equipment, for $1,490 per person.

As I found out on my trip, the Gran Sabana can be dangerous territory. Self-conducted tours deep into the jungle are prohibited by the Ministry of the Environment; tepui climbers are now required to use experienced Venezuelan guides.

WHERE TO STAY: At Canaima Camp, the popular base on the edge of the Gran Sabana, daily rates start at $275 per person ($369 per couple) and include excellent buffet breakfasts and dinners. But most travelers choose package deals that include air fare from Caracas. Accommodation is in simple, clean cabins, and views of waterfalls from the restaurant are spectacular even on cloudy days (of which there are many).

If you select Rudy Truffino's expeditions, you'll stay at Ucaima Camp, a mile or so from Canaima Camp, in a "back of beyond" setting where travelers' tales are a regular topic of conversation.

INFORMATION:

Viasa International Airways, 18 E. 48th St., New York, N.Y. 10017, (212) 586-1489 or 1-800-221-2150.

Avensa Airlines, 800 Brickell Ave., Suite 1109, Miami, Fla. 33131, (305) 381-8001 or 1-800-872-3533.

Consulate General of Venezuela, 111 Water St., Suite 402, Baltimore, Md. 21202, (301) 962-0362. -- David Yeadon