The lava chasm was sixty meters across and of unknowable depth. It occupied the entire floor of a natural intersection chamber where four tunnel corridors met—and beneath the thin crust of cooled basalt that bridged their side of the chamber to the opposite tunnel entrance, the orange-red glow of active magma pulsed and breathed like something alive.
The air above the chasm was twenty degrees hotter than the surrounding tunnel, and the thermal updraft was strong enough that Marcus had to lean into it slightly to stand upright.
The bridge was natural—a pressure ridge of cooled lava that had solidified across the narrowest point of the chasm some unknown time ago. It was perhaps two meters wide at its broadest point and less than a meter at its thinnest, and the surface was irregular: cracked into interlocking tiles of different thicknesses, some clearly hollow-sounding underfoot.
Hollow basalt over active lava meant thermal fracture risk. Step on the wrong tile and it could go through.
Talon assessed the bridge for forty seconds. He moved with the systematic precision of a creature who had done this, or something like it, many times before. His sickled claws—those distinctive curved killing instruments that the Deinonychosauria were named for—were not merely weapons.
On uneven volcanic rock they functioned as crampons, distributing his body weight across five precise contact points per foot rather than the flat-soled impact of human boots. He tested each tile section individually before committing his weight, using one foreleg pressed to the surface to detect resonance before stepping.
Marcus watched the technique for ten full seconds, then applied it himself: forearm pressure on each tile section before shifting body weight. He felt the difference immediately—hollow tiles resonated differently against his palm than solid ones.
"Copy what I do," he said to Reyes and Patel without looking back. "One at a time. Wait until the person in front has reached the next solid tile before you move."
Talon reached the far side in forty seconds. Marcus followed in four minutes, sweating despite the air temperature from the concentration required. Reyes crossed in five minutes, her jaw set and her equipment pack clutched to her chest.
Patel was halfway across when the tile sequence they had all used for the middle section emitted a sharp crack. Reyes grabbed the nearest wall support instinctively. Marcus froze.
Patel went completely still, weight distributed across both feet on adjacent tiles. The cracked tile held. Three seconds passed that felt considerably longer.
Patel moved—carefully, each step a deliberate calculation—and reached the far side. He sat down hard against the tunnel wall and let out a long, controlled breath.
"That," Reyes said, "was the worst forty seconds of my life."
"There are going to be worse ones," Marcus said honestly.
Talon was already moving down the new tunnel ahead of them, echolocation pulse at standard navigation frequency, unconcerned. For him, Marcus thought, following, this is just Tuesday. Marcus moved to the far wall of the ascending tunnel beyond the bridge and sat for three minutes while his hands stopped shaking—a purely mechanical recovery process that his body insisted on regardless of his brain's impatience.
The geological hammer was still in his grip and he became aware of this only when Talon approached, lowered its head to examine the tool with a precise sniff, then looked up at Marcus with an expression that communicated, with remarkable clarity, the raptor's opinion of the hammer's structural integrity relative to the crossing they had just completed.
"I know," Marcus said. "It was never the right tool for the job. But it was the only one we had." Talon clicked once and moved forward.
The tunnel beyond the lava chasm was cooler by four degrees—measurable on Marcus's sensor—the magma fissure below acting as a thermal boundary between the lower and upper cavern networks. They were moving into new geological territory.