Tunnels 01, Tunnels

the excavation site.
    When Will finally appeared, he was walking at a snail's pace down the incline of the entrance tunnel, his shovel resting on his shoulder and his head hung low.
    "Hi, Will," Chester said brightly as he lobbed a whole handful of the clay balls at the defiant bottle. All of them predictably failed to hit the mark. There was a moment of disappointment before Chester turned to Will for a response. But Will merely grunted, and when he did look up, Chester was disturbed by the marked lack of sparkle in his friend's eyes. Chester had noticed something wasn't right over the past couple of days at school -- Will seemed to be avoiding him, and when Chester had caught up with him, he had been withdrawn and uncommunicative.
    An uneasy silence grew between them in the chamber until Chester, unable to stand it any longer, blurted out, "There's a block--"
    "My dad's gone," Will cut him off.
    "What?"
    "He locked himself in the cellar, but now we think he's gone."
    Suddenly, it became clear to Chester why his friend's behavior had been odder than usual. He opened his mouth and then shut it again. He had absolutely no idea what to say.
    As if exhausted, Will slumped down in the other armchair.
    "When did this happen?" Chester asked awkwardly.
    "Couple of days ago -- he had some sort of fight with Mum."
    "What does she think?"
    "Hah, nothing! She hasn't said a word to us since he went," Will answered.
    Chester glanced at the tunnel branching off the chamber and then at Will, who was contemplatively rubbing a smear of dried mud from the shaft of the shovel. Chester took a deep breath and spoke hesitantly. "I'm sorry, but... there's something else you should know."
    "What's that?" Will said quietly.
    "The tunnel's blocked."
    "What?" Will said. In a flash, he became animated again. He sprang out of the armchair and dashed into the mouth of the tunnel. Sure enough, the entrance to the peculiar brick room was impassable -- in fact, only half of the passage still remained.
    "I don't believe it." Will stared helplessly at the tightly packed barrier of soil and stone that reached right up to the roof of the tunnel, closing it off completely. He tested the props and stay immediately in front of it, tugging at them with both hands and kicking their bases with the steel toe cap of his work boot. "Nothing wrong with those," he said, squatting down to test several areas of the spoil from the pile with his palms. He cupped his hand, scooped up some of the earth, and examined it as Chester watched, admiring the way his friend was investigating the scene.
    "Weird."
    "What is?" Chester asked.
    Will held the dirt up to his nose and sniffed it deeply. Then, taking a pinch of the soil, he discarded the rest of it. He continued to rub it slowly between his fingertips for several seconds and then turned to Chester with a frown.
    "What's up, Will?"
    "The props farther into the tunnel were completely sound -- I gave them a once-over before we left last time. And there hasn't been any rain recently, has there?"
    "No, I don't think so," Chester replied.
    "No, and this dirt doesn't feel nearly damp enough to cause the roof to slip in -- there's no more moisture than you'd expect. But the weirdest thing is all this." He reached down, plucked out a chunk of stone from the pile, and tossed it over to Chester, who caught it and examined it with a bewildered expression.
    "I'm sorry, I don't understand. What's important about this?"
    "It's limestone . This infill has bits of limestone in it. Feel the surface of the rock. It's chalky -- totally the wrong texture for sandstone. That's particulate ."
    "Particulate?" Chester asked.
    "Yes, much more grainy. Hang on, let me check to make sure I'm right," Will said as he produced his penknife and, folding out the largest blade, used it to pick at the clean face of another piece of the rock, talking the whole time. "You see, they're both sedimentary rocks, and they look pretty much the same. Sometimes it's quite hard to tell the difference. The tests you can use are to dropacid on it -- it makes limestone fizz -- or look at it with a magnifying glass to see the coarser quartz grains you only get in sandstone. But this is the best method by far. Here we go," Will announced as he took a minute flake of the stone he'd pried from the sample and, to Chester's amazement, slipped it off the blade and into his mouth. The he began to nibble it between his front teeth.
    "What are you doing , Will?"
    "Mmmm," Will