The big scientist kept an eye on his odometer, or mileage counter, while the boys watched for a clearing. It was slightly over a mile before they found one, and Zircon pulled off the road to let Brad Connel and Ruiz go by.

The jeep stopped as the two came abreast and the geologist called, "Want to trade stations?"

"We like this one," Zircon replied with a grin.

"Don't blame you. I have another three miles through this stuff. Well, so long."

The jeep started off and was soon lost as the path curved slightly.

Zircon looked at his watch. "Plenty of time, but we might as well get ready."

A few minutes search disclosed a spot far enough away from the clearing for safety, with no trees to be uprooted by the blast. Zircon took two of the dynamite sticks Rick carried and one of the caps. He placed the cap over one stick and used a special tool, like a jar opener, to crimp it into place.

"This is the only really delicate part of the operation," he said. "If the crimpers slip, they could set off the cap and the dynamite. So be careful when you do it. Keep the crimpers low on the f.l.a.n.g.e of the cap."

He found a rubber band in his pocket and used it to hold the two sticks together. A coil of wire was produced next, and the connection made to the dynamite cap. Zircon dug a shallow hole with his heel and put the dynamite sticks in, then backed off unwinding wire as he went.

The detonator had been left in the jeep. Rick got it and carried it to where Zircon waited with the pair of wires.

"How does this thing work?" Scotty asked.

"It's a dynamo," Zircon replied. "When the handle is pushed down it engages gears that spin a flywheel, which operates the dynamo long enough to send an electrical charge through the wires."

"So don't sit on the handle," Rick joked.

"And don't kick it," Scotty added.

Zircon connected the wires to a pair of terminals on top of the detonator, then looked at his watch. "Plenty of time. We might as well take it easy. Anyone hungry?"

No one was. It was too soon after breakfast. Instead, Rick took the opportunity to ask questions.

"I can understand the general principle of what we're doing, but can you tell us exactly what happens?"

"Sure. When the dynamite charge goes off, it sends shock waves through the earth in all directions. Whenever a shock wave strikes something of different density, its direction and velocity change. For instance, if there is denser rock a few hundred feet down, that will cause a change of both velocity and direction. With me so far?"

"I think so," Scotty said. "The denser the stuff the wave strikes, the faster it moves. Like sound waves. I mean, sound moves faster in water than in air, and faster in a steel rail than in water. Is it the same?"

"Just about," Zircon agreed. "The shock waves radiate away from us, through the earth, and eventually reach the recorders on the other side of the mountain. You can see what happens, I think. Waves will arrive at different times, depending on the path they took and the kind of material they went through."

Rick nodded. "So if there's molten rock, or magma somewhere in the way, the shock wave that goes through it will slow down and arrive at the recorder later?"

"That's it. The tracings we get can be a.n.a.lyzed to give us a kind of cross-sectional look at the mountain. You see, we know how fast the waves travel through different kinds of earth structure. Also, we will know the point of the explosion and the location of the recorder for each shot. Which reminds me. We'd better get out the equipment and locate ourselves precisely."

"How?" Rick asked. "What will we use for landmarks?"

"The top of the mountain, for one, and if you'll look carefully to a point slightly south of east between those two banana palms, you'll see the top of the control tower at the airport."

Rick shook his head. "Good thing you're with us. I completely forgot to watch for landmarks."

"That was the first thing I had in mind in looking for a spot," Zircon told him.

The transit gave a precise angle between the two landmarks. Zircon drew a line on the map connecting the southern tip of the mountain and the airport tower. Then, with that as his base line, it was easy to draw two lines at the correct angles from each of the points. The transit's position was where the two lines intersected.

By the time the scientist had finished, it was nearly one o'clock. The three walked to the detonator. "Pull the handle up," Zircon directed.

Rick did so. "I'll count down from ten seconds. Push down on zero."

It was like the countdown for a rocket firing, Rick thought. Zircon called out the time starting at one minute, then called off the last ten seconds. As he reached zero, Rick pushed the handle home.

The dynamite went off with a roar that sent leaves and dirt flying, and Rick felt the shock wave slam against his ears with stunning force.

"Open your mouth next time," Zircon said. "I forgot to warn you." He was already reeling in the wire. "Let's get going. One mile farther on for the next shot."

At the next station the same procedure was repeated, but before it was time, there was a far-off explosion. Zircon looked at his watch. "Brad Connel. Right on time." In another fifteen minutes there was an even more distant sound as David Riddle's first shot went off. They ate their lunch and listened to the echo off the mountain.

Zircon and the boys were ready when their time came. Location this time had been made on sightings toward the mountain, and a flagpole at Cape San Souci on the western side of the island.

The road petered out and they were forced to go cross-country to reach the third shot station. Fortunately, Brad Connel had left a path of crushed vegetation, so it was only necessary to follow where he had led.

After the third shot, the three collected their equipment and drove back to the hotel.

They were the first back. All three were sticky from the heat, and somewhat insect bitten. By unanimous consent they headed for the showers.

Rick dressed except for his shoes, then stretched out on his bed. He wondered what the day's work would show. The memory of the earthquake was still fresh, and he was anxious to see if it had come from rising magma far below, or from some other source. He had a mental image of white-hot rock rising sluggishly, melting a path to the surface. Now and then the magma struck water, or gas-producing minerals, and then there was a tightly held explosion that made the earth shudder.

Well, it was probably like that, from what he had read about volcanic action. Anyway, he could do without earthquakes. They were unnerving.

Scotty finished dressing, and Rick slipped on his shoes. It was time for the others to be back. Connel should have arrived only a few minutes behind them, but it would take longer for the others because they had gone around the mountain in the other direction.

The boys walked to the staff conference room and found Hartson Brant and Julius Weiss. The two were busy unrolling long strips of paper covered with blue shadings.

"Find anything yet?" Rick asked his father.

"No. We're just getting ready to take a look. How did it go?"

"No trouble. Zircon must still be in the shower. Probably Connel is, too. He must have been right behind us."

The scientists started poring over the traces.

"Here's your first shot," Hartson Brant said. He pointed to where a series of squiggles began. Rick could see nothing of interest. All the pen marks looked about the same to him. It would take expert a.n.a.lysis to make anything out of them.

The boys left the scientists to their work and wandered out into the parking lot. "I want to take a closer look at that crack," Rick said.

"Same here. Suppose it goes to China?"

Rick grinned at his pal. "That's a myth. If you drilled a hole straight down through the center of the earth from here you wouldn't come out anywhere near China. You'd be in the Southern Hemisphere."

"Don't get technical on me, boy."