Chapter 35

When they'd felt the first tremors inside the lava tube, Mac saw Iona turn and take a step back toward the entrance.

Mac stopped him by putting a hand on his arm. He grinned as he did it, not wanting the move to seem overly aggressive.

"You know, son," Mac said, "they shoot deserters in the army."

"Hey, hold on—"

"Just kidding. But you need to relax."

"Relax?"Iona said. "You felt the same thing I did."

"And with your EOD training, you ought to know that quakes aren't our biggest concern in lava caves," Mac said.

"It's why the canisters are here, Sergeant, am I right?" Jenny asked.

Before Iona could respond, Mac said, "There's something else you should know, if you don't know it already. The shock waves produced by earthquakes are largely unable to travel through the air. These caves are structurally sound, at least generally, because most of the loose rocks have fallen during the formation process. So the fact is, even though this place does look spooky as hell, these caves are pretty goddamn solid when it comes to absorbing shock waves like the one we just felt."

"We shouldn't have felt anything at all in here," Iona said stubbornly.

"That just means the magma is on the move," Mac said. "But we knew that already, didn't we?"

Mac knew how much force magma exerted on rocks as it moved through the crust; it was what generated most of the earthquakes in volcanically active areas like theirs. Eventually, the fluid pressure from the rising magma caused cracks in the rocks, the lava making space for itself. These were earthquakes related to ground swelling, and they were rarely larger than a magnitude of 5 and usually less than a 3.

What they'd just felt, Mac explained, might have been a volcano tectonic quake or something else, perhaps a long-period quake, which could be an indicator that the magma had moved into the shallower parts of the volcano. Or it might have been some kind of hybrid of the two. And there were other possibilities, of course, though less likely, including the volcanic tremor he was certain he'd felt the other day at the beach with Lono and the other surfers.

But at this point, none of that really mattered. The only thing that mattered was that the magma was on the move and it had just sent that message powerfully enough that they had felt it even in here.

When he finished with his brief tutorial on volcano-related quakes, he told Iona what their next stop needed to be.

"Really?" Iona asked.

"Really."

"You think that's a good idea?" Iona asked.

"Probably not," Mac said.

"If it's all the same to you, sir, I think I'll sit that one out."

Mac grinned. "Not really a request, soldier."

"All due respect, sir," Iona said, "you're not my superior officer."

"All due respect," Mac said, "I am today."

Now Jenny was the one putting a hand on Iona's arm. She pointed at Mac with her other hand.

"My guy here is a fireman at heart," she said. "When others are running out of the building, he's running in."

"Let's go," Mac said, leading them toward the entrance.

When they were outside, he grabbed his cell phone from where he'd tossed it in the jeep, called Rick Ozaki, told him where they were, and asked him to meet them at the base. As soon as Rick got there, Mac said, they were going to take a ride.

"Do I want to know where?" Rick asked.

"We're going up the mountain," Mac said. "Big Mauna is sending us a message."