Rachel raised another question of significance. "Why was the body even moved?"

Wallace offered one explanation. "Perhaps to keep safe whatever was hidden here. During Malachy's time, England and Ireland were under constant attack by wave after wave of Viking raiders. The island, with no fortifications, would have been especially vulnerable."

Gray nodded. "And if this crypt was where the key was kept, it must be somehow tied to the body interred here. So to preserve the knowledge, both the body and the key had to be moved to a safer location."

"But what the hell is this key?" Seichan asked. "What are we even looking for?"

Gray looked toward the only other clue left to them by Father Giovanni. He moved over to the wall and studied the charcoal notations next to the cross. He laid a hand on the wall. What had Marco been trying to figure out?

The others gathered behind him.

He looked up at the Celtic cross. Only now did he realize something. "The cross," he said, running his fingers down it. "It's made out of the same stone as the sarcophagus. It even feels scoured like the crypt."

Wallace stepped closer. "You're right."

Gray turned to him. "This wasn't put here by Malachy or some other pious Christian to mark the grave."

"It was already here."

Gray looked at the cross with new eyes, not seeing it as a Christian symbol but a pagan one. Did it offer some clue to what the key actually was? From the notations on the wall, Father Giovanni had been trying to figure something out.

Needing to know more, Gray pointed his light at the bottom of the cross. "The set of three spirals near the base of the cross. Is there any special significance to them?"

Wallace moved over to join Gray and Rachel. "It's called a tri-spiral. But it's actually not three spirals. Only one. See how the three of them join and blend to form one sinuous pattern. This same triple pattern can be found marked on ancient standing stones across Europe. And like many pagan symbols, the Church appropriated this one, too. To the Celtic people, it represented eternal life. But to the Church, it was the perfect representation for the Holy Trinity. The Father, the Son, and the Holy Ghost. All entwined together. The three who are one."

Gray moved his gaze up to the single spiral that sat in the middle of the cross, like the hub of a wheel.

He remembered Painter's original briefing about the symbol. How the pagan cross and spiral were often found together, one overlapping the other. The cross was a symbol for Earth. And the spiral marked the soul's journey, rising from this world to the next, like a curl of smoke.

Gray's attention shifted to Father Giovanni's markings drawn on the wall. He sensed some meaning behind the notations and lines. He could almost grasp it, but it remained tantalizingly out of reach.

Stepping closer, Gray put down his flashlight and reached to the circular section on the cross. He ran his fingertips across the scored markings.

Like spokes on a wheel.

As the thought popped into his head, he was still staring at the spiral in the center of the cross. He had compared it earlier to a wheel hub. It even looked like it was turning.

Then suddenly he knew.

Maybe he had sensed it from the beginning, but he couldn't get past the Christian symbolism. Now, considering the cross anew and pushing aside preconceptions, he recognized what was nagging at him.

"It is a wheel," he realized.

Reaching more firmly, he grasped the stone circle and turned it counter-clockwise, in the direction of the curl of the spiral.

It moved!

As he turned the wheel, his eyes shifted to the calculations drawn on the wall. The cross hid a clue about the key, but to unlock it you had to know the proper code. The wheel must act like a combination lock, protecting some hidden vault where the key was once stored.

From all the calculations on the wall, Marco had been working on that proper sequence, trying to figure out the numbers to the combination.

Unfortunately, Gray realized something too late.

You only got one guess at the combination.

And he got it wrong.

A loud boom shook the ground under his feet. The floor suddenly dropped from under him. He grabbed for the cross and hooked his fingers onto the crossbar. Looking over a shoulder as he hung, he watched the back half of the chamber floor rise up. The entire floor was tilting-tipping away from the only exit.

The others screamed and scrambled to brace themselves.