Gray straightened, stunned.

That was it.

That was the one piece he needed.

Inside, a dam suddenly released, understanding flowed through Gray's head. Images flashed across his mind's eye faster than he could follow, but still, somewhere beyond reason, they came together.

He stared up at the roof.

Like a telescope.

He turned and grasped his enemy in a hug. Seichan stiffened, unsure what to do with her arms.

"I know," he whispered in her ear.

She jolted at his words, perhaps misinterpreting them.

He let her go. He dropped to the floor and checked the base of the cross. It sat on a half sphere of bronze. He felt around the edges. It wasn't flush. There was a wafer-thin gap between the stone and the bronze.

He sprang back to his feet and ran for the pack he'd abandoned on the floor. He dumped it out and found a black marker. He knelt down, needing to see it for himself. He worked quickly, his marker flying across the stone.

As he worked, a part of his mind traveled back to Bardsey. He recognized the partial calculations on the wall now. The circle with the lines. Father Giovanni was smarter than all of them. He had figured it out. The circle was a representation of the earth. His notations-

"They were calculations of longitude and latitude."

The others gathered around him.

"What are you talking about?" Wallace asked.

Gray pointed to the bronze sculpture in the center of the room. "It's not a cross," he repeated. "It's a navigational tool. One tied to the stars!"

He finished his drawing.

His sketch showed how the cross could be tilted, how its arm could be pointed at a star, how the weighted sinew could act like a plumb line, and the turning wheel of the device could measure degrees.

"It's an early sextant," he explained.

"Oh my God." Wallace fell back in shock. A palm rose to his forehead. "For the longest time, archaeologists have debated how the ancients were so accurate in positioning their stones. How precisely they were able to align them!" He stabbed a finger at the drawing. "Bloody hell! That device could even be a theodolite!"

"A what?" Rachel asked.

Gray answered, recognizing it now, too. "A surveying tool, used to measure horizontal and vertical angles. Used in engineering."

"The worship of the spiral and the cross," Wallace said. "The symbols truly do represent the heavens and the earth."

Gray stared down at his sketch of the earthbound cross pointed at the stars. "It's more than that. The symbols also represent the worship of secret knowledge, the secrets of navigation and engineering."

Seichan brought them back down out of the stars with a sobering question. "But what does all this have to do with the Doomsday key?"

They all stared toward the bronze cross.

Gray knew the answer. "In ancient times, only the priest classes had access to such powerful knowledge." He glanced at Wallace for confirmation.

The professor nodded.

"To unlock the Doomsday key, we have to demonstrate that same knowledge."

"How?" Rachel asked.

He remembered what Father Giovanni had been calculating at Bardsey. "We have to use the stars above and calculate a navigational coordinate. I'm guessing we have to dial in our location here. An approximate longitude and latitude." He faced the others. "That's the combination."

"Can you calculate it?" Wallace asked.

"I can try."

Gray returned to the floor. The Celtic cross functioned differently from a sextant, which used mirrors and reflections to discern latitude and longitude. But it wasn't that dissimilar.

"I need a fixed constant," he mumbled and stared up at the quartz starscape. It had been put there for a reason.

"The north star," Seichan said. She crouched and pointed to the chunk of quartz that represented the pole star, used over countless ages for navigation.

That would do.

He worked quickly. He knew the approximate coordinates for Clairvaux from using his GPS during the drive here. He pictured the reading from the unit:

LAT 48°09'00"N

LONG 04°47'00"E

Longitude and latitude measurements were broken down to hours, minutes, and seconds. Just sweeps around a clock. Like the lines scored into the spinning wheel of bronze on the cross. It was all proportional.

In under a minute, he had what he believed were the correct assignments using the ancient tool and their current location.