The vault door—Reyes named it that, and the name stuck—was a natural basalt plug two meters thick that blocked the main ascent passage from the primary cavern system to the surface approach tunnels. On the far side of it, based on the acoustic signatures Marcus's sensor was reading through the rock, were at least six mercenaries and the control station for the seismic survey array.
They had been climbing for an hour through progressively shallower passages, guided by Talon's surface-approaching thermal mapping—the raptor could detect the differential between deep geothermal warmth and surface solar warmth by the direction of the temperature gradient, orienting toward decreasing internal heat like a compass needle. But the vault door was the chokepoint.
Marcus spent fifteen minutes with his acoustic sensor pressed against the basalt, reading everything he could through two meters of rock. Six thermal signatures on the far side.
A complex arrangement of equipment—the seismic array's surface unit, based on the mechanical vibration signatures, plus what sounded like portable field communications gear and a generator. The mercenaries weren't retreating. They were still working, still attempting to map and claim the geothermal system even as sections of it were flooding with the magma surge they had triggered.
"We can't go around it," Patel said. "The side passages on this level are all dead ends—I've been tracking the geology for the last thirty minutes. This is the only viable surface approach."
Marcus looked at Talon. The raptor was sitting at the base of the vault door, one foreleg pressed flat against the basalt surface, running a sustained low-frequency pulse through the rock and reading the return. After a moment, Talon looked at Marcus and clicked in a precise sequence: three short, one long, two short.
Marcus had learned enough of Talon's click vocabulary in the past several hours to understand this was a structural assessment: the pattern he had come to associate with "hollow section, mechanical fracture risk." Marcus pressed his own hand against the door at the point Talon indicated—a section roughly sixty centimeters across and thirty centimeters to the left of center. He struck it once with the geological hammer.
The return sound was hollow. A void space behind the plug surface at that location, where the basalt had cooled with an interior air pocket. A structural weakness.
Marcus looked at Reyes and Patel.
"When that goes through, they're going to know we're here," Reyes said.
"Yes," Marcus agreed. "So we have to move faster than they can react."
He looked at Talon. "I need you on the other side of that door about three seconds after it goes. Whatever those mercenaries have planned for you, you are considerably more alarming than I am."
Talon tilted its head at the precise angle Marcus had come to recognize as comprehension. Marcus set the geological hammer, lined up the strike on the hollow section, drew a breath, and hit the vault door with everything he had.
The hollow section gave way in a cascade of black basalt fragments that scattered across the approach tunnel floor like broken crockery. Marcus was through before the dust settled, his eyes adjusting from the darkness of the passage to the work-light illumination of the approach tunnel with painful speed—the fluorescent equipment lighting felt like standing under a Nevada noon sun after six hours in total underground dark.
He was aware, in the analytical background of his mind, that he looked precisely as bad as a person who had spent the previous fourteen hours rappelling down an obsidian shaft, navigating a geothermal prehistoric ecosystem, crawling through volcanic tunnels, surviving a lava surge, and free-climbing a ninety-degree basalt chimney. He was reasonably certain this appearance worked in his favor, combined with the three-meter amber-feathered Cretaceous predator that came through the wall gap directly behind him.