"Hey!" he called. "How about some chow? I'm starved!"

With Chet leading the way, the boys went to the shack familiarly known as "Carpenter's Cottage." There, after a late snack of sandwiches, soda pop and what remained of Aunt Gertrude's cake, Frank called a council of war.

"For the past few days," he said, "the water shortage has been a serious problem, fellows. But now it's becoming dangerous. I hope Bob finds the leak in the reservoir before much more time goes by!"

"You said it," Chet agreed. "But how?"

"All we're sure of," Joe pointed out, "is that Potato Annie and Sailor Hawkins are determined to save their homes and would do anything to keep the water out of the valley."

"I'm convinced that there are more important people than the squatters interested in keeping Tarnack Reservoir from filling," Frank told him. "I think we've got to look for something that ties 65 in with Dr. Foster, the scientist Dad is searching for."

Chet crammed another sandwich into his mouth.

"You moan that secret process Dr. Foster was working on when he disappeared could be tied up in some way with Skull Mountain?" he mumbled.

"Possibly," Frank said, smiling.

"I don't see the connection," Joe disagreed.

"I don't either, Joe-yet," said his brother. "But everything I found out today points to a tie-up between Klenger, Dr. Foster and the thin man called Sweeper. And we saw saw the thin the thin man on the mountain!"

"That's true," Joe nodded. "But what about Bob's theory?" he demanded after a moment. "Bob's still convinced the water is running out through an underground channel."

"I don't believe there is any old tunnel," Chet grumbled. "I poked away at every likely spot along the sh.o.r.e with that old pole and nothing happened."

"Nothing but a ducking," Joe reminded him with a grin.

Chet sniffed.

"There's got to be a tunnel," Frank said determinedly. "There's no other way for the water to escape."

"Uh-huh," Joe scoffed. "But where is it? It doesn't start in the river bed, or the men who built the dam would have discovered it. And Bob and 66 d.i.c.k and Chet have searched along the sides of the reservoir."

"Yes, I know," Frank admitted. Then suddenly a thought struck him. "Wait a minute!" he said excitedly.

"What is it?" Joe wanted to know.

Frank looked at him. "Didn't d.i.c.k say the water rises in the reservoir in the daytime?"

Joe nodded. "But he said it never rises above twenty feet."

"And remember when Bob picked up the shingles after he'd planted them?"

"Sure," said Joe. "It was just before nightfall."

"Then that's it!" Frank exclaimed excitedly.

"That's what?" Chet interrupted.

Frank turned to him. "If there is is an underground channel," he explained, "and I'm betting an underground channel," he explained, "and I'm betting my bottom dollar that there is-it's draining the water off at night! That's why the shingles didn't reveal any currents that would indicate where the water is escaping!"

"Why wouldn't the tunnel drain off the water in the daytime, too?" Joe objected.

Frank shrugged. "There you've got me. But there's one way to prove whether the geologist Bob told us about had the correct theory about an underground channel. That's by planting some stuff in the reservoir at night!"

"And if the tunnel exists, the stuff will be carried 67 through and we can watch for it at the other end!" Joe finished, reflecting Frank's excitement.

"But where is the other end?" asked Chet skeptically.

"According to the book Bob read, the underground river emptied into the bay at Bayport," Frank replied. "But since then the whole coast line in this area has been sinking.

The river mouth could be away out in the bay now."

The boys were silent for a moment, each considering the possibilities of the plan. If it worked, they'd be much closer to solving the mystery.

"Hey!" Chet said suddenly. "I smell smoke!"

The boys sniffed.

"See if something's burning on the stove, Chet," Frank suggested.

Chet rose heavily from his chair and went to the stove.

"Nothing here," he reported.

Chet started back, then stared. Wisps of smoke were curling through the floor boards of the wooden shack!

"Fire!" he yelled, pointing to the floor.

Frank and Joe leaped to their feet.

"Come on, Frank!" yelled Joe. "It's under the floor!"

"Take that bucket of water with you!" Frank ordered, pointing behind his brother.

Joe grabbed up the bucket as Frank ran for the 68 door. The elder Hardy boy pulled on the k.n.o.b, but the door refused to open. He pulled again with all his strength.

"Chet!" he gasped. "Give me a hand!"

Young Morton also gripped the doork.n.o.b, and to-gather they strained at it.

"It must be jammed," Frank breathed. "It won't budge!"

"Try the windows!" Joe shouted.

They ran to the two windows in the shack, then drew back. Flames were already licking the window sills!

Joe emptied the water bucket on them, but the flames continued to mount. The boys looked at one another. They were trapped!

CHAPTER IX.

The Man of the Mountain.

desperately, the boys looked around for a means of escape. Lifting a chair, Frank hammered at the wooden door-but it would not yield.

A tongue of flame shot under the door and licked greedily at the floor. Smoke curled thickly through the cracks in the shack, and a burning shingle dropped from the roof.

The smoke made the boys' eyes water, and they began to cough.

Then, just when it seemed there was no way out, they heard excited voices-and a moment later, the blade of an axe bit through a plank in the doorl "It's Bob!" Joe cried.

"Boy, will I be glad to see him!" Chet spluttered weakly.

The three youths grinned at one another, their eyes shining with joy and relief.

"Grab anything of value you can lay your hands on!" Frank instructed.

The boys scurried around the small room, stacking their arms with clothes, food and engineering instruments.

Blows from Bob's axe had split the wood at the jamb. An instant later, the door was flung back, and the boys ran out into the open. They dropped their bundles and breathed deeply, filling their lungs with fresh mountain air.

Bob and d.i.c.k watched them anxiously.

"What happened?" Bob asked.

"That's what we'd like to know!" Joe declared.

"A fire started under the shack," Frank told the Sail engineer. "And when we ran to put it out, we discovered the door was jammed!"

"It wasn't jammed," d.i.c.k put in. "It was padlocked!"

"Padlocked!" Joe gasped.

"Yes," Bob said grimly. "Someone snapped the lock shut while you were inside!"

"And then set fire to the shack!" Frank added slowly.

"Golly," said Chet. "Who would do a thing like that?"

"Anyone of a number of people who want to drive us out of this region," d.i.c.k replied bitterly.

They stared at the shack. It was completely in flames now, and as they watched, the roof fell in. They waited until the fire died down and the shack 71 was a black, smoldering ruin, then carefully searched the ground around it for a clue to the incendiary's ident.i.ty.

There were several footprints in the soft earth, but none that they could recognize.

Joe set his mouth determinedly as they gave up their search.

"That makes one more score we've got to settle with-with-" He broke off helplessly.

"With whom?" Frank teased him.

Joe grinned sheepishly. "I don't know," he admitted. Then he added belligerently, "But you can bet someone's going to pay for this!"

The boys helped Bob and d.i.c.k carry the articles they had salvaged from the shack to their pup tents. In the s.p.a.ce adjoining, the two engineers pitched tents of their own-part of the camping equipment they had stored in the shack.

Bob told the boys of the white stripe he had painted on a rock to mark the water level of the reservoir that afternoon. When he and d.i.c.k had gone to look at it that night, the level of the water was one foot under the mark. But the rock itself was damp for four feet above the stripe!

"During the afternoon," the engineer stated, "the water rose four feet! But after dark, five feet of water had drained away!"

"Maybe the rock above the mark was damp from the storm," Frank suggested.

"Uh-uh," Bob shook his head. "d.i.c.k and I 72 checked that. The rocks higher up were dry!"

Frank wanted to tell the engineers about the plan to plant articles in the reservoir at night, but he did not wish to arouse their hopes only to have the experiment end in failure. He caught Joe's eye, and the younger Hardy silently agreed not to men-lion the plan.

Early the next day, Frank and Joe set out along one side of the reservoir, keeping a few feet above the water.

Behind them trudged Chet. The latter had again aired his conviction that the underground outlet did not exist, but he did not want to miss out on the excitement in the event the boys found it.

The slope at the water's edge was dotted with rocks, patches of shrubs and creeping vines which extended under the water. Any one of these patches of fallen rocks and foliage, the boys felt, might conceal the mouth of the tunnel.

They prodded the brambles with long sticks, but tearing away the thickly matted branches and leaves was a long, tortuous and hopelessly unrewarding process.

Chet sat down on the ground and wiped his forehead.

"Wow, is it hot!" he declared. He shifted his position, then leaped from the spot where he had been sitting as if he had been shot. "Ouchl" he yelled 73 He put his hand to the seat of his pants and gingerly pulled out a huge thorn. Chet looked at it with disgust.

"That's what I get for letting you two talk me into hunting for that old tunnel!" he declared.

"Wha-at?" Joe challenged him. "Coming with us was your own idea!"

Chet plunked himself down on a flat slab of rock, taking considerable care that it was free of th.o.r.n.y brambles.

"You could've said 'no,' " he pointed out reasonably.

Joe turned to his brother. "What can you do with a guy like that?" he asked.

Frank looked speculatively at the water. "We might duck him," he suggested.

"We might at that," Joe agreed, his eyes lighting up at the idea.

Chet blanched. "Don't you dare!" he pleaded, attempting to wriggle to his feet.