In September 1924, the wheels of a truck sank into the ground behind the Pelham Courts apartments in Washington, D.C. On investigating, the building’s manager and janitor discovered a mysterious brick-lined passageway that led to a bizarre network of concrete tunnels extending as much as 32 feet underground.

The discovery put Washington into two days of wild speculation. Was this a German plot? A relic of the Civil War? But then Smithsonian Institution entomologist Harrison G. Dyar came forward to admit that he had dug the tunnels when had lived in the capital 10 years earlier. From Modern Mechanix, August 1932:

Dyar’s obsession had begun innocently enough. He told the Washington Star that in 1906 he had dug a flowerbed for his wife, and “When I was down perhaps six or seven feet, surrounded only by the damp brown walls of old Mother Earth, I was seized by an undeniable fancy to keep on going.” He had continued the project in secret for 10 years, stopping only in 1915, when he moved out of the area.

“I did it for exercise,” he told the New York Times. “Digging tunnels after work is my hobby. There’s really nothing mysterious about it.”

(Thanks, Allyson.)