Another favorite pastime was digging a tunnel under his home; he attributed the secret of his success to "visits by elves" while he worked in the tunnel: "While I'm digging in the tunnel, the elves will often come to me with solutions to my problem."
Wikipedia cited a rather entertaining Time article as a citation for this.
A couple years ago someone on r/TwinCities posted about finding a boarded up tunnel in their basement. Turns out that it went to the neighbor's basement.
Apparently they aren't that unusual in certain neighborhoods with houses of a certain age - they were used historically to move coal for heating the houses.