We were still processing what she said when a broad surge of water erupted from a tunnel not far away. It quickly swamped the entire work zone. The murky water reached our knees, and then was waist-high. It reflected the light of our headlamps to shine indistinct patterns on the rocky ceiling. Wooden beams stained black with coal dust floated by, and miners’ helmets and lunch boxes…. When the water reached my chin, I instinctively held my breath. Then I was entirely underwater, and all I could see was a murky brown where my headlamp shone, and air bubbles that sometimes floated up.

“Mine floods have many causes. Whether it’s groundwater, or if the mine has dug into a surface water source, it’s far more life-threatening than a flood above ground,” the teacher said over the sound of the water.

The holographic water vanished and our surroundings returned to normal. Then I noticed an odd-looking object, like a big metal toad puffing out its stomach. It was huge and heavy. I pointed it out to the teacher.

“That’s an anti-explosion switch. Since methane is a highly flammable gas, the switch suppresses the electric sparks that ordinary switches create. That’s related to what we’ll see next, the most terrifying mining danger of all …”

There was another loud crash, but unlike the previous two times, it seemed to come from within us, bursting through our eardrums to the outside, as huge waves contracted our every cell, and in the searing waves of heat, we were plunged into a red glow emitted from the air around us that filled every inch of space in the mine. Then the glow disappeared, and everything plunged into darkness.

“Few people have actually seen a methane explosion, since it’s hard to survive one in the mines.” The teacher’s disembodied voice echoed in the darkness.

“Why did people used to come to such a terrible place?” a student asked.

“For this,” the teacher said, holding a chunk of black rock into the light from our headlamps, where its innumerable facets sparkled. That was the first time I saw solid coal.

“Children, what we just saw was a mid-twentieth-century coal mine. There were a few new machines and technologies after that, such as hydraulic struts and huge shearers, which went into use in the last two decades of the century and improved conditions somewhat for the workers, but coal mines remained an incredibly dangerous, awful working environment. Until …”

It turned dull after that. The teacher lectured us on the history of gasified coal, which was put to use eighty years ago, when oil was nearly exhausted and major powers mobilized troops to seize the remaining oil fields. The Earth was on the brink of war, but it was gasified coal that saved the world…. We all knew this, so it was boring.