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But that drone stuff isn't actually about certainty, it's about guesswork. Good solid builder science is the only way to be certain about anything. Measurements, observations, calculations. It occurs to you that the air pressure down here is higher than on the surface by 20%, meaning that you're about 1500 meters below sea level, and that you can hear another watercourse about 5 meters away joining with the outflow from the mirror. You also use your compass to find that the mirror is on the east wall of this room. You rapidly weave a 3-dimensional map of the course you took down through the Land of Bedrock and Rust, add in the relative position of the room you're in now, and confirm that following the water back up won't take you more than a few hours since you can fly now.

Then you sit down, because whoah. Certainty is a hell of a drug.