Are creature environmental effects a bubble or column?

I am planning out my campaigns first BBE and am going with an aboleth.

In the rules an aboleth has a number of regional effects that reach up to a mile. This aboleth is currently in an underground pool in a mine about a mile underground. So would the characters see any evidence of the aboleth effects at ground level away from the mine, or would the effects only exist in a bubble the top of which is level with the top of the mine?

To my mind the effects will be a bubble, so widening out from the aboleth, if the approached underground they would meet them far sooner?