A couple Sundays ago, Nate Swain was standing in the field of gravel under the Zakim Bridge with a big rake-sled thing that he had custom built for his purposes. The spot, which is part of Paul Revere Park on the Charlestown side of the placid Charles River, turns out to be one of the most restless places anywhere.

It sits amidst the endlessly rushing traffic of Interstate 93 and its various ramps above. Plus there are trains idling at North Station, conveyor belts and cement trucks of neighboring Boston Sand & Gravel, and now and again a siren warning that the railway bridge is going to creak open.

Nate Swain's “Zen Under the Zakim" in 2015. (Greg Cook/WBUR)

Swain is an artist, based in Boston’s North End. And on this particular afternoon, his purposes were to again work on the secret, guerrilla Zen rock garden that he’d created in the shadow of the bridge in 2013 and has been occasionally tending since with the help of friends. He raked the gray stones into seven giant, concentric rings.

When I stopped by a couple days later, there was a splash of white stones in the center ring, with some larger, darker stones stacked in the middle, and on top a playing card, the ace of hearts.

“Someone added that playing card since I did it,” Swain told me later.

Zen rock gardens, with their raked gravel or sand, are renowned for their tranquility and calm and contemplativeness. This place is the opposite of that. It’s a place so literally buzzing with activity that it can be hard to hear oneself think.

“There’s this sort of odd thing of being in this opposite space of where you’d usually find something like that,” Swain says. “It’s the most challenging space to find that calm. … If you can find calm when it’s really noisy, then it’s easier to find calm when it’s quiet.”

You may have seen Swain’s art around Boston without realizing it. In 2009, he got permission to cover the bricked up windows of the NStar substation at the corner of Prince and Salem streets in the North End with photo-murals. The image for each window was built by digitally collaging together five to 10 photos. The pictures create the illusion of looking through curtained windows, with cats perched on sills, to rooms decorated with vases and potted plants.

The following year, Swain covered the wall of the Assaggio restaurant facing St. Leonard’s Peace Garden on Hanover Street in the North End with a photo-mural depicting a leafy, vine-covered wall that he had photographed at the Arnold Arboretum.

Behind the statues at St. Leonard’s Peace Garden, you can see Nate Swain's photo-mural depicting a leafy, vine-covered wall that he had photographed at the Arnold Arboretum. (Courtesy)

Swain has also painted murals of forests, rivers, giant dandelions and fields of sunflowers at the old Bartlett Yard bus maintenance facility in Boston’s Roxbury neighborhood, at the Arsenal Mall in Watertown, on a building on Boston’s Beacon Hill.

“I just want to make people smile,” he says. “I want to add color. I see it as a gift. The city needs more fun. It’s slowly getting more fun, but it’s way behind other cities.”

Nate Swain's “Low Tide City” or “Barnacle City." (Courtesy)

Then the land under the Zakim Bridge caught his eye. “I worked driving a tour trolley in Charlestown, and I drove over that bridge every day to go to work, and looked down,” Swain says. “I went down there not even knowing what I wanted to do.”

At the edge of the Charles River, near the North Washington Street Bridge, by the dock in front of the Residence Inn by Marriot on the east end of the park, he’s been assembling “Low Tide City” or “Barnacle City.” It’s “a little city” of bricks and stones that disappears under the river and appears when the tide goes out. "I realized it could be an art piece about sea level change,” he says. “People could watch it flood and imagine Boston could do that if sea level rises.”

And right under the Zakim Bridge, Swain realized he could rake the existing expanse of gravel he found there into patterns, much like a traditional Zen rock garden, to create “Zen Under the Zakim.” He says, “If you really sit there and you listen to all the noise, some of the traffic, even though it’s really noisy, it does sound like ocean waves.”

But he admits, “I don’t find calm there. I’m doing that piece mostly for fun. I try to find places where I can do art without asking permission. In Boston, there’s so much bureaucracy. There’s no room for spontaneity. … With all the bureaucracy and the permission-asking, it sucks all the energy and all the inspiration out of the art piece itself.”

Raking the stones. (Courtesy)

Last Sunday, I wandered over to see the secret Zen rock garden again. It was a mild, sunny afternoon. Some skaters were doing tricks on the curbs and stone benches under the bridge. At Swain’s guerrilla garden, I found the playing card was gone and some vehicle had left tracks through the gravel rings. Swain figures a police cruiser drove through. It’s a risk of making art in public—especially without permission. “I’m trying to train my brain to work on something and let it go,” he tells me.

“I have this theory,” he adds, “if you put something up beautiful and colorful and fun, in good taste, uplifting, it will stay and everyone will love it and no one will bat an eye.”

Greg Cook is co-founder of WBUR's ARTery. Follow him on Twitter @AestheticResear or on the Facebook.

Nate Swain's “Zen Under the Zakim" in 2015. (Greg Cook/WBUR)

Nate Swain's “Zen Under the Zakim" in 2015. (Greg Cook/WBUR)