The next time you find yourself walking across the corner of Maiden Lane and Broadway in Lower Manhattan, remember to look down. In front of what used to be the William Barthman Jeweler is something unexpected: a working clock, embedded in the sidewalk like a precious gem.

Installed in 1884, this clock stood as an unusual advertisement to draw customers to William Barthman's store. Now, it's one of the oldest landmarks in downtown Manhattan—and one of the most overlooked.

I recently visited the people who maintain this asphalt aberration, who told me that an estimated 50,000 people walk over the clock every day. In order to accommodate such an intense amount of wear, the jeweler gets a custom glass cover made from United Silica—a manufacturer in Franklin, New Jersey, that also builds high-stress materials for GE.

The pane on the clock can handle up to 2,000 lbs of pressure and needs to be replaced around every four years due to intense wear. Here's what the old, tarnished glass lenses look like once they're removed:

William Barthman moved from the corner store that used to be in front of the clock years ago. Now, the company is based just a few doors down. But the clock is still a central part of their lore—inside the company's basement sit dozens of books that contain newspaper clippings referencing the clock and store, some dating back more than a century.

We descended into the basement to look at the clock, seen by so many commuters over the years, from below.

Surprisingly enough, the clock itself looked far less complicated than I had imagined. The only sign that anything was unusual about it? The light filtering through its gears—and the muffled bump of every footstep that landed on the fuzzy glass above. [William Barthman]