Vintage photos of:
September 26, 1924. Washington, D.C. "Mysterious tunnel." A strong Hardy Boys vibe here. National Photo Company Collection glass negative. View full size.
UPDATE: While initial speculation (bootlegging, espionage) was soon dispelled by an eccentric insect expert's explanation that he had dug the passages "for exercise," historical evidence suggests that this was a tunnel of love. Or at the very least, bigamy.