Unease welled up in the lieutenant colonel.
He glanced at the Dome Wall to his left.
It was one of the load-bearing structures around the Underground City's perimeter, cast from high-density reinforced concrete.
It was over three meters thick, with an additional waterproof layer and a rock-anchor support system around its exterior.
By design standards, it could withstand at least two close-range detonations of Bunker Buster Nuclear Bombs.
But now, dust was constantly falling from the wall designed to be "impregnable."
Some chunks of stone had already begun breaking loose.
A four-man fire team was stationed closest to it.
Their leader was a private second class named Brian, a Kentucky native who had enlisted less than two years ago.
He was less than five meters from the wall.
The shaking grew more violent, and dust fell in great sheets, making him cough.
"Hey, did you guys hear that?" Brian turned his head toward the soldiers beside him.
"What?"
"There seems to be some kind of noise inside the wall."
Brian frowned and listened for a few seconds.
Beneath the rumbling, he faintly heard another sound.
It was low and deep, like... something gnawing.
No, that wasn't right. It sounded like grinding!
Like a gigantic drill crushing rock, but mixed with an organic scraping sound that belonged to no machine.
The sound made his hair stand on end.
"Reporting!"
Brian shouted toward the command platform, but his voice drowned in the growing roar.
After hesitating for a second, he held his rifle ready and walked up to the wall.
Up close, the sound became much clearer.
"Crrk... crrk, crrk..."
The sound of concrete being stripped away and pulverized layer by layer by some force was right beside his ear, less than half a meter away.
And it was getting closer!
"Fuck..."