why tunnels are maximum in round shapes?

Question

what could be the reason for this tunnels in round shape ? is it related to any gravitaional force ? please answer me. thanks in advance

maximum in the sense i said almost. because i recently heard that some rectangle tunnels also exists.

Answer 1

You need to be more explicit about what you mean by maximum. In terms of enclosing the largest area with a certain width of material, a circular area shape is the answer.

Answer 2

maximum what?