By the time the sun had crawled off one of Hidalgo's spike like mountaintops, Doc and his men were ready for departure.

Doc had taken his usual two-hour exercise long before dawn, while the others still slept.

After that, Doc had awakened his men, and they had all seized brushes and quick-drying blue paint, and gone over their entire plane. The ship was now blue, the sacred color of the Mayans!

"If the inhabitants of this mysterious Valley of the Vanished think we're riding in a holy chariot," Doc had commented, "they may let us hang around long enough to make friends."

Ham, waspish and debonair, carrying his inevitable sword cane — for he had several of them — offered jocosely: "And if they believe in evolution, we can arouse their interest by passing Monk off as the missing link."

"Oh, yeah?" Monk grinned. "Some day you're gonna find yourself in a pile that will pass for hamburger steak, and you won't know any more about who done it than you do about who framed that ham-stealing charge on you."

Red-necked, Ham twiddled his cane and had nothing more to say.

Gasoline for twenty hours' flying reposed in the tanks of the big tri-motor speed plane.

Doc, in the control bucket, turned the radial motors over with the electro-inertia starting mechanism. He let the cylinders warm so there would be no such unpleasantness as a cold motor stopping at a critical moment in the take-off.

Out across the lake, Doc ruddered the plane. He rocked the deperdussin type control wheel. The floats went on step — skimming the lake surface. Then they were off. Doc banked about and headed directly for the most rugged interior region of Hidalgo.

It was Doc's own idea, borne out by Johnny's intensive study of the country's topography, to use pontoons instead of landing wheels on the plane. Due to the wildly rank jungle and the unbelievably craggy nature of the region, chances were one in a thousand of finding a clearing large enough for a set-down.

On the other hand, Hidalgo was in a sphere of great rainfall, of tropical downpours. The streams were small rivers, and here and there in a mountain chasm lay a tiny lake. Hence the floats on the plane.

While Doc lifted the plane to ten thousand feet to find a favorable air current, and thus cut gasoline consumption, his five friends used binoculars through the cabin windows.

They hoped to find trace of their enemy, the blue monoplane. But not a glimpse of its hangar did they catch in the nodular, verdurous carpet of jungle. It must be concealed, they reasoned, somewhere very near the capital city of Blanco Grande. But they didn't sight it.

Below was an occasional patch of milpa, or native corn, growing in jungle clearings. Through the glasses, they could see natives carrying burdens in macapak, or netting bags suspended by a strap about the forehead. These became scarcer. Where had once been milpa patches was only a thick growth of uamiz bushes ten to twenty feet high. They were leaving civilization behind. Hours passed.

Great barrancas, or gorges, began to split the terrain. The earth seemed to tumble and writhe and pile atop itself in inconceivable derangement. Mountains lurched up, gigantic, made black and ominous by the jungle growth. From above, the flyers could look down into canyons so deep their floors were nothing but gloomy space.

"There's not a level place down there big enough to stick a stamp on!" Renny declared in an awed voice.

Johnny laughed. "I told Monk that Columbus tackling the Atlantic Ocean had a pipe compared to this."

Monk snorted. "You're crazy. Us settin' in comfortable seats in this plane, and you call it somethin' hard! I don't see nothin' dangerous about it."

"You wouldn't!" Ham said dryly. "If we should be forced down, you could take to the trees. The rest of us would have to walk. And a half mile a day is good walking in that country under us!"

Renny, up in the pilot's well with Doc, called: "Heads up, you eggs! We're getting close!"

Renny had checked their course figures again and again. He had calculated angles and inscribed lines on the map. And they were nearing their destination, the tract of land that was Doc's legacy! It lay directly ahead.

And ahead was a mountain range more nodular and sheer than any they had sighted yet. Its foothill peaks were like stone needles. To the rampant sides of the mountains clung stringy patches of jungle, fighting for existence.

The great speed plane bucked like a plains cayuse as it encountered the tremendous air currents set up by the precipitous wastes of stone below. This in spite of Doc's masterful hand at the controls. An ordinary pilot would have succumbed to such treacherous currents, or prudently turned back.

It was as though they were flying the tumultuous heart of a vast cyclone.

Monk, hanging tightly to a wicker seat, which was in turn strapped with metal to the plane fuselage, had become somewhat green under his ruddy brick complexion. Plainly, he had changed his ideas about the ease of their exploration method. Not that he was scared. But he was about as seasick as man ever became.

"These devilish air currents explain why this region has not been mapped by plane," Doc offered.

Four or five minutes later, he leveled an arm. "Look! That canyon should lead to the center of this tract of land we're hunting!"

The eyes, all of them, followed Doc's pointing arm.

A narrow-walled gash that seemed to sink a limitless depth into the mountain met their gaze. This cut was of bare stone, too steep and too flintlike in hardness to support even a trace of green growth.

The plane careened closer.

So deep was the gash of a canyon that twilight swathed the lower recesses. Renny, keen of eye and using binoculars, advised: "There is quite a stream of water running in the bottom of the canyon."

Fearlessly, Doc nosed the plane down. Another pilot would have banked away in terror from those malicious air currents. Doc, however, knew just how much his plane could stand. Although the craft might be tossed about a great deal, they were all as yet quite safe — as long as Doc's hand was on the controls.

Into the monster slash of a chasm, the plane rumbled its way. The motor thunder was tossed back in waves from the frowning walls. Suddenly air, cooled by the small river rushing through the cut and thus contracting and forming a down current, seemed to suck the plane into the depths. Wheeling, twisting, the speed ship plummeted among murky shadows.

Monk was now a striking example of the contention that sudden danger will cure seasickness — for he was entirely normal again.

Doc had the throttles against the wide-open pins. The three radial motors moaned and labored, and the exhaust pipes lipped blue flame.

The progress of the craft along the chasm was a procession of leaps and drops and side-whippings, as though they were riding an amusement-park jack rabbit, or roller coaster.

"It'll be a long old day before another gang of white explorers penetrate into this place!" Renny prophesied.

Doc's arm suddenly leveled like a bronze bar.

"The Valley of the Vanished!" he cried.

QUITE suddenly, it had appeared before them — the Valley of the Vanished!

A widening in the strange, devilish chasm formed it. The valley had roughly the shape of an egg. The floor was sloping, of such a steepness that to land a wheel-equipped plane on it would be an impossibility.

There was only one spot of comparative levelness, and that was no greater than an acre or two in area.

It was on this level spot that the eyes of Doc and his five men instantly focused. They stared, unbelieving.

"Good Heaven!" gasped Johnny, the archaeologist.

From the little flat towered a pyramid! It adhered in a general way to the architecture of the Egyptian type of pyramids, but there were differences.