It’s called The Altar, The Wall, The Rock Spot and even Shrinetown. Locals speculate that it’s the remains of an old mill or some ancient Indian ruin. An unconfirmed source says that the Army Corps of Engineers named it an “historic dam abutment” in their recent Wissahickon Creek Feasibility Study.

But what is it really?

The two men who created it just call it “The Spot.” They built it alongside an undisclosed tributary of the Wissahickon creek between 2004-2007. The only tools they used were shovels. “It’s the equivalent really of graffiti,” says S, one of the artists behind the project. He should know, he’s also a familiar Philadelphia street artist, who is also known as Radius. Although the project isn’t destructive, it was certainly never sanctioned by any official body, and the artists wish to remain semi-anonymous.

I visited The Spot with S and co-creator R on a recent spring afternoon. As we approached from a small path, we noticed a man was sitting trailside directly above it. Invisible from the trail, he had no idea what was tucked into the hill directly in front of him. We exchanged greetings and turned down a path to the creekside.

The simplest way to describe The Spot is as a series of terraces built into the Wissahickon Schist bedrock. With that said, what R&S built is not only impressive in design and ambition, but nearly flawless in execution. Using available stone washed into the adjacent creek, they fit their walls and terraces together like a puzzle. It’s an organic design built for human activity. There’s a long bench curved around a fire pit. There are stairs that lead to different levels and single chairs that emerge from in spots in the stone. The power of place that they coaxed from the hillside is as impressive as the engineering effort that went into building it.

“We’ve met and heard of a lot of artists and writers that have said that they come here for inspiration.” says R.

I asked if they had a plan when they started on the project. “I had a vision of the first floor,” says S, “…but everything else is R’s idea. It was a constant process of me being like I think it’s finished and he’s like, no let’s do this!” While neither had any direct professional experience before the project, both have plenty now. S works in landscaping and R does stonework professionally. Both are native Philadelphians and have been coming to this section of Fairmount Park since they were kids.

As The Spot has become better known, R&S have launched other projects in the Wissahickon. An ambitious plan to build a “winter shelter” in a grove of white pines was abandoned for a smaller scale project. Called “The Horseshoe,” the artists used stones excavated directly from the site to build a small arc of a wall/bench. Pine needles make the ground soft. Shade has encouraged moss and ferns to grow on the stone.

Down in the floodplain, next to a marshy expanse of skunk cabbage, they’ve built a large stone bench in a small clearing. In another area of the park, they’re at work building what they describe as a teepee.

Sanctioned or not, The Spot is built to last. It’s already survived Hurricane Irene and nearly a decade of severe seasonal flooding. Light maintenance has been needed, but its been minimal. Whatever future generations call it or wherever they think it came from, it will remain for them to enjoy.