Dig the right spot. Then get out of the way.
Every cave is full of boulders and crystals resting on soil. Dig under one and it wobbles for a moment, long enough to see it coming, then down it comes. Step aside in time and you get a big GULP! Don't, and Rasmus is flat as a pancake, a little ghost in a hard hat floats away, and he is straight back for another go.
Deeper down, old timber props hold up whole ceilings of boulders and crystals until you knock them out, and dry-stone walls only open with a stick of dynamite. All the while the groundwater rises from below: no clock to read, just water you can see coming. It is so clear you still see every stone under it, and it splashes and ripples round Rasmus, his cap lamp shining through it.