Code (Virals #3) by Kathy Reichs, Brendan Reichs

The Virals are put to the ultimate test when they find a geocache containing an ornate puzzle box. Shelton decodes the cipher inside, only to find more tantalizing clues left by "The Gamemaster." A second, greater geocache is within reach—if the Virals are up to the challenge.

But the hunt takes a dark turn when Tory locates the other box—a fake bomb, along with a sinister proposal from The Gamemaster. Now, the real game has begun: another bomb is out there—a real one—and the clock is ticking.