Barlin estimated the distance according to the light intensity from down below.

He could sense that the source must be at least thousands of feet deep just then, and might have been built upon the ice cap at that depth, the bottom of which was the lithosphere of the South Pole.

And the thought left him a little depressed.

While prolonged exposure to extreme darkness wouldn't improve eyesight, it would certainly increase sensitivity to light by a hundredfold.

That was certainly the case for