"Rig the plane floodlights so they cover the whole floor of the canyon," he ordered. "And step on it. It's gonna be dark before long."

A man said: "Here is that ball of string you wanted."

"After it gets dark," Spad ordered, "stretch that string between you when you stand guard. If anybody should try to crawl past you, there's a good chance of them hitting the string."

"Right."

"Now get that fellow Renny out here."

There was a scuffling sound, and Renny said, "Holy cow!" twice, and a man yelped in agony. Doc took a chance and looked. Renny was tied hand and foot, but it was taking four of them to hold him.

Spad Ames drew a revolver and c.o.c.ked it.

"I don't know whether Doc Savage is dead or not," he said, "but in any case, I don't think we're going to need you."

DOC SAVAGE lifted up and threw a smoke grenade. Then he threw a gas grenade, another smoke, and kept pitching them. The grenades were waterproof. The gas grenades weren't noisy. The smoke ones were. They sounded louder than shotguns. Violence of their hatching hurled the black smoke yards in every direction. Each resembled, an instant after it exploded, a large black octopus; this soon turned into a cloud.

Doc went into the smoke. A revolver smashed out deafeningly-Spad Ames' gun, probably.

"Renny!" the bronze man called.

Renny answered in Mayan, a language almost unknown to the civilized world, which Doc and his men used when they did not wish to be understood. Renny was safe, lying to the right.

Doc had a pocket knife out when he reached Renny. He slashed ropes, got the big-fisted engineer free.

"Can you run?" he asked.

"I hope to tell you!" Renny boomed.

Doc Savage spoke in Mayan. "Go up the canyon. Follow the river. And now-hold your breath!

There's gas in this smoke."

Renny went away.

The smoke had spread. Wind would soon sweep it away. Doc tossed more smoke grenades, into the wind where the sepia pall would be carried over them.

There was some shooting. Three or four men were coughing. Others cursed. Doc felt about for the planes.When he found one of the ships, he felt in his pocket and brought out one of the rocks which had a sharp edge. He struck it against the undersurface of one of the wings. Wing skin split. So did the fuel tank.

Gasoline flooded down his arm.

There was a small waterproof lighter in one of his pockets, and he got it with the hand which was not gasoline-drenched. The high-test aviation gas caught with a sound that might have been a giant coughing.

Doc had fixed the position of all three planes in his mind. He made for another one. Spad Ames had become silent-ga.s.sed, probably. There were plenty of men left for yelling and shooting, however.

Firing the second plane was managed without incident, but when he ignited the third craft, flames seized his arm, made it a bundle of red pain. He ran a few yards, pitched into the sand, and smothered the fire before it did more than scorch his skin.

Suddenly, it was lighter. Wind was sweeping the smoke away. He had no more of the grenades, so there was nothing to do but run for it. He wanted to locate Spad Ames, if possible. He did try, but failed, and had to run for it.

He made the creek safely, and a few minutes later he joined Renny.

Chapter XII. THE KEY.

LONG TOM was standing watch, some distance from Monk, Ham and the Colorados. They startled Long Tom, and after he had apparently tried to jump out of the canyon, he grinned at Renny and said: "This was once I never expected to see you again, you big lug."

Renny rubbed his jaw, pondered, then said: "Holy cow!"

That was the extent of their reunion greetings. They moved on toward the spot where Monk and Ham were keeping the Colorados.

Soon they could hear Monk and Ham. The pair were not speaking in loud voices, but they were saying unusually violent things to each other, until they saw Renny and greeted him.

The joyful reunion was soon over. Habeas Corpus and Chemistry bounced around in glee.

Renny walked over to Ruth and Mark Colorado. It was getting dark fast; even the reflection of the sunlight on the canyon rim was dying. Renny stared at the prisoners.

"Spad Ames and Locatella know you got these two," he said. "Saw them in the plane. They're not happy about it."

"What have you learned about this mystery, Renny?" Monk asked.

"Well, practically nothing. I know why Spad Ames was so anxious to get hold of the Colorados."

"Why?"

"Hostages. He was going to use them as human shields for himself. Mark and Ruth Colorado-and I got the idea that is just a name they took for themselves-seem to be very important personages in this place that we've heard mentioned as behind the mists."

"You don't know what they mean by that silly business-behind the mists?"

"No. Your guess would be as good as mine.""Spad Ames is plenty anxious to get back into the place."

"Yes. I gather that. It is something pretty tremendous that he is after."

MONK was not too enthusiastic about leaving the watching of pretty Ruth Colorado to the others, but he accompanied Doc, at the bronze man's suggestion.

They went to a point below the source of the river several hundred feet, where the canyon was very narrow.

"Good place to make a stand, if Spad Ames should try to reach us," Monk said.

"They are too heavily armed for us," Doc a.s.sured him.

"Then what're we here for?"

He found out. Doc produced a metal tube which was filled with round dark objects which somewhat resembled shotgun shot. He sprinkled these over the sand carefully.

"I get it," Monk said. The stuff was an explosive which was not affected by moisture, and detonated from pressure-an improved variation of the ordinary Fourth of July spit-devil. An intruder could hardly pa.s.s this point without stepping on the stuff; immediately there would be a loud, if not very damaging, explosion. Enough noise to give a vociferous alarm.

This, Monk gathered, was to be their precaution for the night. They could get some sleep. Personally, he needed it.

It was dark when they got back. Intensely black. Doc Savage bent over the Colorados, as if testing the ropes that tied them.

He deliberately let his pocket knife drop against Mark Colorado's leg-as though it might have been an accident.

Five minutes later, casually, as if he had just found it out, he said: "My knife seems to be missing."

"You used it to cut my ropes," Renny reminded.

"I might have dropped it there," Doc remarked, as if dismissing the matter.

For the next few minutes, Doc listened carefully. He decided that Mark Colorado had found the knife, was using it to slyly cut himself loose.

"Let's get over here a minute, where the Colorados cannot hear," Doc said, "and discuss the campaign plan."

They a.s.sembled fifty yards away, and the bronze man said: "Don't get excited. The Colorados are escaping right now, probably."

"What!"

Monk howled, so loud that fifty echoes came back from the surrounding stone pinnacles.

"Sh-h-h, stupid!" Ham admonished.

"Holy cow!" Renny said, trying-to whisper. "What's the idea?""We will trail the Colorados," Doc explained.

"How?"

"Back in Monk's laboratory in New York, I splashed some chemical over both Mark and Ruth Colorado. At the time, I intended to use the stuff to locate the Colorados in the city, but as it developed, we didn't have a chance to do that. We can use it now."

"What kind of chemical is it?" Renny demanded. "And how-"

"Phosph.o.r.escent," Doc told him. "That is, it gives off light, but not visible light. It is infrared phosph.o.r.escent."

"Not visible light?"

"Unless you view it through a fluoroscopic eyepiece," Doc elaborated. "You cannot see X rays ordinarily, you know. But a fluoroscope makes them visible through the fluorescing characteristic of certain materials."

The description was a little too general for Long Tom, who liked his explanations to be specific. "As far back as l883, a man named Becquerel studied the invisible region of the solar spectrum, with reference to the quenching of phosph.o.r.escence by red and infrared light, and he discovered-"

"Kindly spare us, if you don't mind," Renny interrupted. "Right now, I don't feel like hearing a scientific discussion of how and why it works. If it works, swell."

Doc Savage produced a pair of spectacles from a pocket. He broke them at the bridge, kept one lens himself and handed the other to Renny. "Hold that in front of your eyes, and you should be able to distinguish the faces of both Colorados, as a pale spot of light. That is, if they are visible from here."

Renny peered and squinted. "Holy cow!"

"But it has been hours and hours since that chemical was dumped on the Colorados," Ham muttered.

"It does not wash off readily," Doc advised.

Renny took another squint through the small fluoroscopic eyepiece. "If you fellows think the Colorados are escaping," he said, "you're wrong."

They went back. Mark and Ruth Colorado had not moved. Apparently the ropes which held them had not been tampered with.

Doc said nothing. He thumbed on a flashlight, took one of the black arrowheads from a pocket, looked at it thoughtfully, then put it back. He knew both Colorados watched him.

IT was near ten o'clock when Mark Colorado threw the ropes off his wrists and ankles, and approached Doc Savage. The bronze man grew tense, thinking of the knife. The blade was short, but it did not take a long blade to cut a throat. He set himself for defense.

Mark Colorado felt very cautiously, located the pocket in which Doc had put the arrowhead. He took the arrowhead, nothing else.

Both Colorados then crept away in the darkness.

Doc breathed: "You fellows awake?"They were. "I been layin' here with a rock in each hand," Monk whispered.

"Come on," Doc said.

They trailed Mark and Ruth Colorado. The infrared phosph.o.r.escent chemical was so pale that they could hardly distinguish it.

"We're gonna have to improve that stuff," Monk whispered. "How did you figure on using it in New York, Doc?"

"By equipping watchers with fluoroscopic spectacles sensitive to the stuff, and planting them at all train gates, bus stations and airports," Doc explained. "That way, they could spot the Colorados, even if they did dye their hair and alter their clothing."

The Colorados went straight to the spot where the river poured out of the aperture in the base of the cliff.

"Holy cow!" Renny breathed a moment later.

The river had stopped flowing.

Doc Savage went forward suddenly. The others followed. They stood, a moment later, at the mouth of an aperture which was perhaps two and a half feet high and seven or eight feet wide-the hole out of which water had stopped pouring. They learned that much by exploring with their hands.

They listened. Sounds came out of the hole. Noises that would be made by two people climbing.

Monk whispered: "I'm goin' in there! It's a secret entrance, or something!"

With Monk, to get a violent idea was to act upon it. He dropped to all fours and scuttled into the hole.

He did not get far.

Came a gurgling roar of water. Also a louder howl from Monk. The river again spouted out of the hole.

Ham unlimbered a flashlight, fanned the beam, then yelled: "There goes Monk!"

The homely chemist was going head over heels. He squawked each time he bounced off a rock, flailed his arms and howled: "Grab me, somebody!" The water had carried him out of the hole with terrific speed; it rushed him fifty yards before he practically bounced out on the bank.

Monk glared at Ham and said: "You laugh, and I'll take that sword cane of yours and ram it down your throat!"

"What happened?" Ham asked innocently.