I STILL REMEMBER the moment I got the underground bug — the itch to find out what lies beneath the streets of this town. It was the mid-1990s, and a Pioneer Square antiques dealer was bending my ear about filthy alleys, stinking doorways and City Hall’s perennial failure to provide better facilities for answering nature’s call.

“So have you seen the underground restrooms?” he asked. “Huh?” I replied. “Mind the till,” he told his assistant, grabbing a ladder and flashlight and striding off toward Pioneer Place, where First Avenue and Yesler Way meet.

There, beside the iron and glass pergola, he popped a manhole cover and dropped the ladder. I went down — and gasped. It was a loo fit for a palace, or at least a fancy steamship: tiled walls and floors, porcelain and nickeled brass fixtures, stalls of white Alaskan marble with louvered hardwood doors. Even caked with dirt and cobwebs, it put to shame the solitary toilet showcased in the nearby underground tour.

This “comfort station,” with its 61 plumbing fixtures, women’s lounge and men’s shoeshine room, was proclaimed “the finest underground facility of the type in the United States” when it opened in 1908, just in time for the Alaska-Yukon-Pacific Exposition. When the press decried its cost (the proposed $12,000 project eventually cost more than $24,000), Parks Commissioner Ferdinand Schmitz offered to remove it at his own expense if citizens weren’t satisfied. They were.

Today, the pergola, whose columns served as the restrooms’ vent pipes, is a Seattle icon and designated National Historic Landmark. The undesignated landmark below might be lost to view forever. When it came due for renovation in 1948, city officials opted to close it instead, and paved over the stairways to it. Since my impromptu visit, they’ve welded the manhole shut and, according to Underground Tour owner Sunny Speidel, “thrown beams down there” to support a cement pad above. A similar comfort station built under Westlake in 1917 and closed in 1963 has been buried entirely.

Since then, I’ve lamented not getting a picture — where were phone cameras when we needed them? — and wondered what else might lie beneath the cobbles and concrete we trod each day. Finally, I got a chance to find out, accompanied by the intrepid Ellen M. Banner, a photographer who can’t resist a chance to haul her cameras into dark, buried places. Do you have the bug, too? Grab your shovel, and check your headlamp. We’re going down.