The first thing that the Sadhu asks is for you to venture as far as possible into the sky. This can be accomplished by building a pillar to the build limit and then attempting to build further. After you return, he asks you to dig downwards and see if there is a limit on downward travel as well. This can be easily accomplished if you have already dug a hole using a Quarry or if your mineshaft is deep enough to have reached the lowest possible bedrock layer. Once you reach the lowest possible bedrock layer, attempt to mine the bedrock (even for a fraction of a second) and the quest will be complete.
Upon returning to the Sadhu, he is deeply troubled by the symmetry of the world. He rewards you with 128 reputation, 4 experience points and the ability to move onto the next and final quest in the series, The Book of the Sadhu.