"I got addicted to archaeology," said Mathes, who earned the nickname Digger as a fresh-faced Northeast Philly kid with an unbridled enthusiasm for the hunt. "I just wanted to find more stuff. Not specifically just arrowheads, but anything I could find."
Now 33, Mathes calls himself a 'privy pirate.' He and his friends dig up backyards and empty lots within historical boundaries, searching for 200- or 300-year-old outhouses. They don't congregate out front of houses, they dig behind them.
The holes are their stoops.
Without running water or regular trash collection, Philadelphians used to dig privy pits and fill them with anything unwanted: hand-blown glass bottles, broken pottery, poop.
Generations of treasure hunters started digging, separately, in the late '70s. Now many travel in groups, searching for historical castaways in Baltimore, New Jersey, and as far as Ohio.
The growth of the Internet and social media has been a double-edged sword, helping people like Mathes find a community that shares his passion, but at times saturating the market for certain objects. Craigslist and eBay have no shortage of 19th Century ceramic plates nowadays.
Mathes sets up tables at flea markets and displays the best stuff at auction houses. Everything sells, he said, but not for millions. A typical glass bottle can sell for $1, but the right ones can go as high as $6,000, he said.
Philadelphia, settled by Swedes in the 1640s and formally founded by William Penn in 1682, is popular with the pirates. And it's different from other key cities along the Northeast corridor. Washington, D.C., for example, mandated shallow holes. In Philly, the outhouses can go as deep as 30 feet. In Pittsburgh, as deep as 50 feet.
Deeper pits mean harder digs – but also more potential for treasure
"Philly has very unusual privies," he said. "So, when you get into one of these, the potential is huge to find a lot of stuff. And the history, of course."
Getting permission from homeowners, Mathes said, can be one of the biggest challenges.
They've knocked on doors over the years, but the easier bets are construction sites, where the excavation is already done and the path to the bottom is already laid out.
Local laws required homeowners to clean out the pits, so the pirates often come up empty. But when they hit upon a full one, the loot can be remarkable. Mathes has found a violin, a whip, and a silk top hat. He's uncovered fingernail clippings, clumps of human hair, a human skeleton. (He took the skull, and left the rest.)
The deepest he's gone was 40 feet, in the outhouse of a former church at 20th and Fitzwater in Southwest Center City. (He didn't find much in that one.)
Those who have been doing this for a while can pick up clues from the privies themselves. The older pits are wood-lined, the newer ones have stone or brick.
"The Northeast was farm country," he said. "There were no laws, so people could dump anywhere on the banks in the woods. I walk up and down the banks of Pennypack off the side roads, and there's dumps all over the place."
To the novice, they might be hard to spot. "They're hidden. Covered up with ash. They actually hid these dumps back in the 1800s so nobody could see them. [They were] actually thinking about the future."
There's a certain thrill to the urban treasure hunt. But it's also dirty, and draining. Between the time and the money and the lack of consistent inventory, few dig full-time.
You have to have a passion for this," he said. "There's a lot of energy involved, and not much money. The amount of time you put into it, you can make more at McDonald's."
Mathes works a day job in construction, but the digging is a labor of love. He and his friends feel more kinship with the Philadelphians of the past than those of today, and after a long day on the job, heading to the pits – their stoop – that feels like home.