All three were startled as a voice suddenly broke in over the wall intercom. It was the operator on duty at the plant's communication center.

"Turn on your TV, skipper," the operator suggested. "We've just had a news bulletin that an earthquake tremor has been felt over in Medfield. There's a big plant there that makes rocket nose cones. A mobile TV crew's been rushed to the scene in a helicopter and they're trying to pick up the action with a television camera."

"Good night! Another quake?" Bud gasped.

Tom had already rushed to the videophone. Flicking it on, he switched to a commercial channel. Soon a picture appeared on the screen. It was a panoramic shot of a landscape, evidently viewed from a hovering aircraft, with a large industrial plant just below.

A TV commentator's voice was reporting developments. "Few visible signs of a tremor," he said. "As you can see, the rocket-plant personnel and the people of Medfield are making desperate attempts to evacuate. Fortunately, most of them have already left the immediate area."

A few cars and trucks could still be seen speeding along the ribbonlike roads within view of the hovering television camera.

"Oh—oh!" The commentator's voice broke in again. "Notice that tall stack just over the plant—see how it's starting to tremble!... It's beginning to crumble!... This must be it!"

Suddenly the whole scene seemed to explode. Plant buildings collapsed like toy houses built of cards, while at the same time huge rocks and trees were uprooted as a yawning crack opened in the ground below.

The three watchers in Tom's office stared in horrified dismay. But a moment later the picture on the TV screen became jerky and distorted, then faded out completely.

After a brief interval, a studio announcer came on. "The relay transmitter must have been knocked out by the quake. We return you now to our regularly scheduled program, but will keep you informed as bulletins come in."

"Great balls o' fire!" Chow gulped as Tom turned off the set. "I sure hope all o' those poor folks in cars got away safe!"

Tom rushed to a wall shelf and pulled out a book on geology. He leafed quickly to a section dealing with known earthquake faults and the distribution of quakes. When he looked up at the others, his face was grim.

"What's wrong, skipper?" Bud asked tensely.

"That quake," Tom replied, "wasn't in a patterned zone any more than the Faber one was!"

Chow's jaw dropped open in a comic look of dismay. "You mean this here ole earth we live on is gettin' all busted up an' twisted around inside?"

"I wish I knew, Chow!" Tom paced worriedly about the office. "It just seems queer to me that both of those quakes should have destroyed vital defense factories!"

On a sudden impulse, Tom snatched up the telephone. His two companions listened as he put through a call to the FBI in Washington. Within moments, a friend at the Bureau, Wes Norris, came on the line.

"Look, Wes," Tom said, "is there any chance this quake that just happened at Medfield and the earlier one at Faber Electronics might have been caused by underground H-bomb blasts?"

"As a matter of fact, we're checking on that very possibility," Norris replied. "In other words, sabotage. Things are pretty hot around here since that news on Medfield came in, so I can't talk much right now, Tom. But I can tell you this," Wes concluded, "we are investigating, and I do mean thoroughly!"

Bud and Chow were shocked when Tom reported his conversation with the FBI agent.

"Brand my rattlesnake stew!" Chow exploded. "Any ornery varmint that'd cause an earthquake ought to be strung up like a hoss thief!"

"I agree, Chow," Tom said. "But how do we find out for sure?"

After closing time at the plant, Bud drove home with Tom. Both Mrs. Swift and Sandy were upset as the boys discussed the situation.

"Tom, if this was deliberate," Mrs. Swift pointed out, "Enterprises may be next on the enemy's list!"

Tom did his best to allay his mother's fears, but inwardly he himself felt apprehensive. Any large-scale sabotage plot would be almost certain to include Swift Enterprises, America's most daring and advanced research center.

When his mother went upstairs to her room, Tom suggested to Bud that they drive to the nearby State Police post. Here he confided his fears to Captain Rock, an old friend of the Swifts.

"You have some request in mind?" Captain Rock inquired.

"How about making a search for any signs of suspicious digging or underground activity in the vicinity of Shopton?" Tom said. "There would have to be an excavation of some sort in order to set off an underground blast."

Captain Rock mulled over Tom's suggestion. "Sounds like a big job, but I'm afraid you're right, Tom. We can't risk a similar disaster here."

"We'd better move fast, too," Bud put in. "Those two quakes so far came only a day apart!"

Rock picked up the telephone and barked out orders. Within half an hour, several carloads of troopers were covering the outlying roads that converged on Shopton. Firemen and Chief Slater's town police force were also pressed into action. They would search every cellar in town for signs of recent digging.

Bud rode in one police car and Tom in another as a house-to-house search was conducted along the highway that ran past Enterprises.

At one weather-beaten house, where Bud stopped with a state trooper, an old man came to the door.

"What you fellers prowlin' around for?" he asked.

"Bomb emergency," the trooper said laconically. "We have orders to search every house cellar for underground openings."

Grumbling, the old man let them enter. He followed them down a rickety stairway. A moment later Bud stumbled and gave a yell. The trooper swung around just in time to see Bud drop from view!

CHAPTER V

SECRET CACHE

As the trooper's flashlight stabbed through the cellar gloom at the spot where Bud had disappeared, there came a loud splash! The light showed a round hole in the floor, rimmed by a low circle of brickwork.

"What's that hole?" the trooper snapped at the owner.

"What does it look like?" the elderly man snapped back. "It's an old well."

"A well!" the trooper exclaimed as he rushed to the spot. "And not even covered? What're you trying to do—kill people?"

The old man sniffed. "Used to be covered, but the lid's gone. Didn't expect to have a bunch of nosy fellers pokin' around down here!"

The state trooper muttered angrily under his breath as he shone his flashlight into the well-shaft. Bud was splashing around below, soaked and chagrined by his accident.

"Give me a hand!" he called up.

The trooper reached down, but was barely able to touch Bud's finger tips. To make matters worse, the sides of the well were slippery with moss.

"Get a rope," the trooper ordered the old man.

"Ain't got one."

The policeman reddened and stood up to his full six-foot-two. "Look, mister—what's your name?"

The elderly man shrank back, as if suspecting that the trooper's patience might have been tried too far. "Ben Smith," he mumbled.

"Okay, Mr. Smith, you get a rope or something else to pull this boy out. And fast!"

Ben Smith gulped on his chewing tobacco and hurried off. A minute or so later he