(Washington Post Staff Illustration)

The hornets almost made me turn back.

I was standing beneath the Oxon Cove Bridge, an unremarkable stretch of the Anacostia Freeway that crosses over Oxon Creek just before it empties into the Potomac, on the border of Maryland and Virginia. They hovered perniciously over my head, their nests blanketing the bridge’s underside. They guarded their domain with long stingers, buzzing with bright yellow wings that seemed like a warning for interlopers to stay away. To get to safety on the other side, I’d have to dash across a slope of jagged boulders, with the slightest misstep sending me tumbling into the water.

This was the halfway point in my expedition. I could turn back, I told myself, picturing a retreat through the marsh and forest that got me here. And yet, for reasons not quite known even to myself, I placed a sneaker on the rocks.

All of this for a glimpse of history, I thought. And I ran.

If you’ve lived in Washington long enough, you might have heard of the Boundary Stones. These knee-high stone markers were placed along the early borders of D.C. beginning in 1791, an order from George Washington himself to outline the country’s brand-new capital. There were 40 of them originally (some have been lost to damage and replaced with replicas), spaced along one-mile intervals in the District’s initial diamond shape, before Virginia took back its contribution to the federal territory in 1846.

Since then, urban life has sprung up around the stones. The country’s first-ever federal monuments now can be found in neighborhood parks, people’s front yards and, in one case, a McDonald’s parking lot. But one stone remains more hidden than its brethren, still resting in a swampy, remote locale that’s probably not too different than how it looked back in the 1700s.

“The last of the nomads,” Stephen Powers calls this particular stone, designated “SE9” for its placement along D.C.’s southeast border with Maryland. Powers is chairman of the Nation’s Capital Boundary Stone Committee and runs BoundaryStones.org, which details each boundary stone’s history and location, with directions on how to find them. He’s been visiting and restoring the stones for the past 15 years.

Searching for SE9 sounded adventurous to me, maybe a fun outdoor jaunt before the end of summer. But when I called Powers to get his advice, he cautioned that SE9 was not for the faint of heart.

“It’s definitely not trail-accessible,” he told me. “You’re bushwhacking to get there.” It’s best to venture there in the winter when the vegetation was less dense, he said, and warned me that I should go with a guide, someone who’s been to the stone before. I should have listened.

I started my expedition to SE9 one sunny Saturday morning, parking my car about two miles away from the site, in a shopping-center parking lot in Glassmanor, Md. From there, I wandered down Oxon Hill Farm Trail, a pleasant, if a bit overgrown, paved path running along the creek before it reaches the Potomac — the only part of the journey that would follow a defined trail. Just as the paved path forked inland, I had to veer off course into the formidable brush, an area marked, naturally, by an abandoned shack.

In the not-too-far-off distance, I could see the Oxon Cove Bridge, which is part of Interstate 295. Following Powers’ directions, I knew SE9 was on the other side of that highway, along the Potomac shoreline. At that point it was time to, in Powers’ words, bushwhack.

Trudging through these backwoods, there were thorn bushes and downed trees and other flora-based obstacles. There were flies and mosquitoes and what I can only assume to be an innumerable amount of ticks. There was at least one snake; a fat, black one that slithered into a pile of rocks as I approached. There were lagoons of watery garbage reminiscent of the trash-compactor scene in Star Wars.

The gauntlet was only about half a mile, yet took more than an hour to navigate. And then, upon reaching I-295, came the dash across boulders at the bridge’s base. My footing didn’t falter, even as the hornets patrolled above. And then came the hard part.

SE9 “defies one to find it,” wrote amateur historian Edwin Darby Nye, in a 1972 paper about the boundary stones for the Columbia Historical Society. “The eight-foot fence that parallels the interstate highway only adds to the difficulty and the mass of thick vegetation makes locating this stone virtually impossible.” In case you were wondering, the answer is no: I did not read Nye’s warning before setting off.

In this primitive peninsula cut off from the outside world by I-295, I found much the same obstacles as the first part of the journey, only with the added elements of verticality and directionlessness. The terrain here angled up and down so sharply that I never truly knew what was going to be just a few yards ahead of me. I had to double back at dead ends made from impenetrable brambles, crawl under ropes of entangled vines, and wade through murky marshland.

All of this would have been easier if I knew exactly where I was supposed to be going. Preservationists have moved SE9 three times since it was first placed, to save it from the receding shoreline. Its original location, long since underwater, was known as Fox Ferry Point, the landing of a Civil War-era ferry line that ran across the Potomac from Alexandria.

Powers’ directions to the stone’s current location instruct one to follow the river’s shoreline south for about 1,000 feet, before cutting back inland to find the stone (“If you are on a small sandy beach with car tires and debris, you are in the right place,” reads his website). But the shoreline is so eroded — and the presence of debris so ubiquitous — that trying to follow written directions through this primordial morass seemed useless.

Luckily, I had GPS. At the top of the many muddy hills along the route, my phone grasped adequate positioning, and I gambled that SE9’s marker on Google Maps was accurate enough to point me in the right direction. So I soldiered on, wiping spider webs from my face until I reached the crest of a hill overlooking a small, sunlit meadow of waist-high greenery.

I wandered into this eerily tranquil grove and turned a slight corner around a tree. There, it appeared like a mirage: SE9, that ancient piece of rubble, encased by a tall black fence. “Jurisdiction of the United States” read its faded engraving. I pictured Benjamin Banneker, the free African American astronomer who, legend says, laid on his back at night to watch the stars move overhead, calculating where to plant each stone more than two centuries ago. The SE9 seemed like a strange relic from a lost time, a bold monument forgotten in the woodland. And it was.

After three hours of exploration, I lingered only for a minute or two. The day was getting hotter, the insect buzz around me louder. So I left the stone to continue its rest out here in the wild. And I tried to find my way back.