A shell casing fell, end over end, half a mile down a chute, to clatter to the floor at his feet. The Spartan paused momentarily to check above for enemies, then, finding nothing to kill, moved on.

This far underground, Nathaniel-007 was invisible to his enemies. The downside to that was that he was just as invisible to his allies, and if he was unfortunate enough to run into a problem, he would have to rely only upon himself to make it back to the surface alive. Possible issues to watch for ran through his mind, from undetected enemy movement to seismic activity or, worse, shaft collapse. Every side-shaft he passed, he checked. Every corner he came to, he took the time to take cover at, and then follow his weapon through, just in case an enemy contingent was concealed within.

This level of vigilance, while natural to him, was tiring. It was four days since he'd gotten so much as a wink of sleep. The only thing keeping him awake was the copious amounts of chemicals – from caffeine to wake-up stims – coursing through his body. Nate ignored the little tremors in his hands, sure in the knowledge that they would disappear with the adrenaline of battle, and until then, they were nothing more than a minor irritation.

More pressing was the burning in his legs. It was from four days of non-stop running. He was moving faster than a fast march, but not quite double time, using Spartan standards to define his speed. Using Marine standards, he knew he was off the scale. That kind of speed was not something even a Spartan could maintain forever. It was not unusual for Nate to spend days at a time at a fast march, but almost double time was unheard of.

The complete silence was driving him crazy. In his experience, silence was always followed by bad luck. If it was quiet enough for him to hear his own near-silent footsteps, something terrible was about to happen. Here, it was so quiet, the silence swallowed his footsteps entirely. After days of such terrible, deafening silence, his nerves were on edge, to the point where he would have jumped at his own shadow, had he had one.

Save your energy, Nate, he told himself, settling back to a fast march, then slowing to parade march. If his legs gave up on him now, he would be entirely unable to defend himself in the unlikely but possible event of an enemy discovering his presence.

A rumpled pile appeared in the tunnel ahead, barely visible even to Nate's sharp eyes and the advanced night vision system in his HUD. He switched to thermal vision, then blinked as the pile burned bright white, and finally, for the first time in four days, he stopped moving forward. Weapon trained on the warm something, he cautiously took a step, and then another step. He stopped again, unwilling to go any closer lest it be hostile.

"Identify yourself," he growled, voice rough from lack of use.

The pile moved, then morphed into a vaguely human shape. "Human. Tory! Lost. Help me?"

It was a child. Nate watched it look around in the darkness, trying to see who had spoken. It stood, or rather, tried to, and after much effort and a few squeaks of pain, managed to lean against the wall in what was the closest thing to a standing position it was capable of. Judging by its size, Nate thought it was around nine years old, and its voice was male.

A boy. Civilian? Must be. "How in the world did you get down here?"

"Fell."

"Down the shaft?" Nate heard the disbelief in his own voice. No normal human could fall half a mile and survive. The physics involved were simply mind-blowing. At terminal velocity, for which a person only had to fall six hundred feet, the damage to the human body would be mortal. The lucky ones would die instantly. The unlucky would suffer for hours, but die anyway in the end.

"Yea."

"Bull. Not even a Spartan could survive that fall unprotected. Doesn't matter. Let's get you topside." He had a mission. A very important mission. Nate frowned, dreading the penalty for ignoring his orders, but he had been trained to protect civilian lives, and these tunnels were not safe for anybody. Much less an injured civilian boy. The civilian was his priority.