On the first day there was the Mystery: On or around Sept. 25, 1924, a truck fell through the earth near 21st and P streets NW, revealing an elaborate, multilevel tunnel underneath. What in blazes was it?

“Old Tunnel Here Believed to Have Been Used by Teuton War Spies and Bootleggers,” read a headline in The Washington Post. Speculation was rampant.

But speculation can last only so long. On the second day, people didn’t want the Mystery; they wanted the Explanation. And that’s what the Reporter was in search of. He left behind the scrum-milling at the cave-in near Dupont Circle and headed across town to find the previous owner of 1512 21st St. NW.

In the red-brick house at 804 B St. SW, across from the National Museum, he found the answer. The tunnel-maker confessed. He was a Smithsonian scientist named Harrison G. Dyar. A scientist who studied bugs. An entomologist.

An entomologist was not as exciting as a Teuton war spy. An entomologist was not as interesting as a bootlegger. An entomologist was boring.

Harrison G. Dyar's obsession with digging tunnels.

But journalists are nothing if not inventive. As he rode the streetcar back to The Post newsroom on E Street NW, the Reporter turned the story over in his head, like a squirrel looking for the weak point in a nut. He was searching for just the right angle and by the time he met the questioning gaze of his editor, he had one: The fact that the tunnels were constructed by someone so boring made them more interesting.

As the Reporter wrote in The Post after interviewing Dr. Dyar: “Throughout the narration one could sense the romance of it — this meek, mild-mannered scientist devoting his time and study to butterflies and moths in a government office in the day and secretly digging underground passage ways in the evenings.”

Even Dyar’s reason for building the tunnels was boring: He said he did it for exercise. Some men play golf, he told the reporter. I dig tunnels.

He had used one boring pastime as a metaphor for another.

Dyar was 58 in 1924. “He wears a beard which is rapidly graying to match his hair,” the Reporter wrote. “His frame is slight, and he is a bit stooped.”

Dyar explained that there was really nothing secret about the 21st Street tunnels. His son Otis used to play in them; neighborhood boys, too. He had worked on them for 10 years excavating, shoring up, dumping the dirt in a vacant lot nearby, facing the walls in brick. Dyar had left the tunnels behind when he moved from the house around 1914 to live out west for a few years before returning to Washington.

Then the Reporter asked an obvious question: Did you ever dig any more tunnels?

The answer was a resounding yes. Underneath the side yard on B Street SW was just as elaborate a bunker, reaching some 24 feet deep. These tunnels, too, were tall enough for a man to stand in and wide enough to walk two abreast. An electric wire snaked through the tunnels, providing pools of light in the inky blackness. Some shafts went straight down and were lined in concrete, with horizontal iron pipes arranged as ladder rungs. The ceilings were arched, like some medieval catacomb. In places Dyar had sculpted the heads of animals and humans.

One arch was inscribed with a bit of Latin: Facilis Descensus Averno. From Virgil, it means: “The way down to the lower world is easy.”

Dyar had started these particular tunnels as a way of getting from the basement to the ash can on the street without having to walk outside. But as at the Dupont Circle house — where Dyar’s tunnels were the metastasized outgrowth of a simple gardening project — he’d just kept going.

In fact, Dyar had dug so deeply on B Street that he’d hit the water table and had to stop. You could sit on one of the tunnel’s lower steps and toss pebbles into the darkness, listening as the stones plopped into the water below.

Was this all really just a hobby done for exercise? Or was the urge to dig a mania, a manifestation of inner turmoil? The Reporter didn’t press Dyar.

In fact, Dyar was far from meek or mild-mannered. He was strong-willed and arrogant. So arrogant, in fact, that if the Reporter had bothered to walk into the dusty labyrinth of The Post’s morgue, bothered to check his own newspaper’s clips, he could have read how just 10 years earlier the “meek” bug doctor had been at the center of a most scandalous affair.

While Dyar had been digging underground, above ground he had been spinning a breathtakingly audacious web of deceit.

To read previous columns, go to washingtonpost.com/johnkelly.