“There are fossils all over the place, and most people don’t know about them,” Barr says. “But once you know what to look for, they’re everywhere.”

You won’t find them by digging — most of D.C.’s fossil-bearing stones eroded away millions of years ago. But you can find the preserved remains of long-dead animals from every geological period, and from all over the world, in the stone-clad walls of the city’s many grand buildings.

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“Because it’s the national capital, people have wanted to have impressive architectural displays here, and so they are willing to pay for expensive decorative stone,” Barr says.

Since 2002, Barr has dedicated hundreds of hours to tracking down noteworthy fossils. Once he’s found an interesting impression, he tracks down the origin of the stone around it, and consults with scientists about the animal it represents. He then posts his findings on his website, dcfossils.org. (There’s also a map.)

“It’s a great resource,” Northern Virginia Community College geology professor Callan Bentley says of the website. “The quality of his research is just amazing.”

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Plus, the site opens people’s eyes to the stunning fossil record all around us, Bentley says.

“Learning that there are fossils in the floor, in the walls, in the dividers between the bathroom stalls — this is a profound realization, that you are surrounded by traces of the ancient world,” Bentley says.

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Here is a guide to five of Barr’s favorite finds.

Lincoln Memorial

2 Lincoln Memorial Circle NW

On the steps flanking the main staircase between the memorial and the reflecting pool, look for a scattering of cobblestones marked with straight, parallel lines that look like soda straws. These burrows were made by underwater animals that lived on the shores of what is now western Maryland about 530 million years ago, during the Cambrian period. Scientists don’t know exactly what these creatures were, but we do know they lived on a planet that would be nearly unrecognizable to us today. The land was mostly barren — a jumble of rocks with occasional slicks of photosynthesizing bacteria. The sun was dimmer, the moon loomed large and the air would have left you gasping for oxygen. The oceans, however, were teeming with tiny animals, including the burrow engravers, that eventually gave rise to the great variety of life we know today. “These fossils are so small, but they are still remarkable because they were the homes of these unknown creatures living on a beach half a billion years ago,” Barr says.

National Gallery of Art

Fourth Street and Constitution Avenue NW

In a first-floor room just to the right of the main atrium, find the golden triptych — a sort of open cupboard — featuring a painting of the Madonna and Child by Nardo di Cione. Then look down and to the left. On the floor near the corner is a circular shape in pink and white that looks as if someone set down a coffee cup without a coaster. What you’re looking at is an ancient nautiloid — a creature that resembles a squid stuffed into a shell and that lived about 450 million years ago, during the Ordovician period. This creature, and others like it that appear in pink, gray and white limestone throughout the first floor of the museum, swam freely in a shallow sea over what’s now Tennessee. These top predators could grow to be more than 11 feet long and used their sharp beaks to pierce the armor of early crustaceans and other prey. “Some of the fossils here have been warped, since these stones underwent enormous pressure, but not enough to totally deform the fossils,” Barr says.

Mitchell Park

23rd and S streets NW

Look at the outward-facing walls on the southwestern corner of the park and you’ll find gray and white shell shapes pressed into some of the stones. These were brachiopods, shelled creatures that dominated the ancient seas but are less common today. The brachiopods that left their mark here probably lived during the early Devonian period, a little less than 400 million years ago, in a coastal area that now makes up the Appalachian Mountains. “This park has been renovated many times, and at some point they put in one of the best fossil displays in the city,” Barr says

International Finance Corporation Building

2121 Pennsylvania Ave. NW

Check out the tan stone that clads this 1997 building by Michael Graves. It holds fossil confetti, remnants of creatures that lived in the shallow seas that covered Germany 160 million years ago, during the Jurassic period. Look for Life Savers-like shapes and rectangles — these are two different cross sections of crinoids, animals also known as sea lilies. All that’s left of them are their stalks, but they once had flower-like plumes, used for filter feeding. The crinoids shared the ancient oceans with giant crocodiles and ichthyosaurs — bizarre, dolphin-like reptiles. “You can find crinoids in limestone all over the city, but what makes the ones in the IFC building special is that you can see entire columns of them still together,” Barr says.

Unification Church

1610 Columbia Road NW

Peek through the fence at this former Mormon chapel now owned by the Unification Church (founded by Sun Myung Moon, famous for mass weddings). The walls are covered in a tan stone dotted with ovals — marks made by algae in a Utah lake between 58 million and 65 million years ago. The algae formed lime deposits, layer by layer, around objects like twigs or shells at the lake’s bottom. These so-called oncoids were buried and flattened, and eventually became limestone. Ancient algae may not spark your imagination, but consider the animals that might have drunk from that freshwater lake: all sorts of little rodents and maybe a few of the first hooved mammals. “This was right after the mass extinction of the dinosaurs, when mammals were beginning to take over,” Barr says.