The Pentagon 911 Memorial. (PAUL J. RICHARDS/AFP/GETTY IMAGES)

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Nearly 10 years on, airplanes still appear in the distance, vaguely at first, then clearer, then louder, then cringingly closer, wailing past the Pentagon.

Truckers still grind gears on Route 27, commuters still honk and rev and skid to rubber-burning stops in the clotted traffic.

But the human ear possesses special gifts, and, somehow, in that two-acre plot of ground called the Pentagon Memorial, especially if you really try, the ear can filter out all that noise and latch onto the sound of peace. It gurgles in the bubbling pools beneath 184 benches, the symbols of 184 lives lost on that day in September. Close your eyes, and listen to the water. Peace.

And then it stops.

Stops cold.

Does it every day.

Every day at 9:37 a.m.

One minute.

The pause feels like a challenge, a subtle admonition, jarring you, nudging you to think about what happened here at that very moment on a sunny morning in 2001 when a Boeing 757 turned into a weapon of mass destruction.

In its three-year life, this space — the first national Sept. 11 memorial — has become a place for the tourist and the mourner who isn’t intimidated by logistics. It is “not easy to get to,” says Thomas Heidenberger, a retired airline pilot whose wife, Michele, was a flight attendant aboard the plane that slammed into the Pentagon at more than 500 mph.

Most visitors pile out of Metro trains on the opposite side of the Pentagon and snake through parking lots to the other side, walking past one sign after another warning them against taking photos, a prohibition that ends when they arrive at the memorial. It has also become a destination for schoolchildren, with the creation of educational programs for children as young as kindergartners. Open 24 hours a day, seven days a week, the site draws about half a million people each year, memorial officials say.

The memorial — one of the most compelling in a city packed with memorials — makes the visitor work to figure it out. It is far from a literal expression, not like the Iwo Jima Memorial, which explains everything in a glance. Instead, it’s an abstraction, pushing visitors to sort out its meaning and decide how to relate to it.

Here is a group of teenagers, advancing tentatively through the thicket of benches. Three boys plop onto a cantilevered bench, pull out their water bottles, place them on the granite surface.

“It’s a cemetery!” one of the girls screams, her face reddening, every inch of her tensing. “Not a table!”

Lisa Leonard, a retired Army colonel who volunteers as a docent, has heard it before. Once it was German tourists. They thought it would be disrespectful to sit on the benches. “But it’s not a cemetery,” Leonard told them. Not a cemetery, at all. Go ahead and sit.

Leonard, who was working in the Pentagon on Sept. 11, 2001, likes to scramble into the planting bed that rings the memorial and pull back the fronds to show visitors the top of a simple, unadorned concrete wall. It starts three inches above ground level — one inch for each year of the too-short life of Dana Falkenberg, a passenger on American Airlines Flight 77, the youngest victim of the terrorist attack on the Pentagon. The wall rises in step with the ages of the victims, Leonard will say, cresting at 71 inches to honor John Yamnicky, a retired Navy captain who was on the same plane.

Tom Heidenberger, who was one of the original directors of the fund that created the $22 million memorial, keeps finding serendipity amid the symbolism. The memorial benches are organized in rows corresponding to the years the victims were born. Heidenberger noticed that his wife, coincidentally, was born in the same year, 1949, as Charles Burlingame, the captain of Flight 77. A captain always sits at the head of the plane, Heidenberger said one afternoon on the phone, and it just so happens that Capt. Burlingame is at the head of his wife’s row. It seems fitting.

So does the placement of the benches commemorating the Falkenberg family. Heidenberger noticed that Dana, the 3-year-old, is honored next to her sister, Zoe, whose bench is in the next row across from Dana’s, even though they were five years apart. Sayre Pono, a student from Maine, walked along Zoe’s row.

“Life goes on,” his teacher, Larry Ross, tells him — that’s why you build a memorial: to honor the dead and inspire the living.

Heidenberger also noticed that the benches honoring Zoe’s and Dana’s parents, Charles Falkenberg and Leslie Whittington, also ended up directly across from each other because he was born in 1956 and she in 1955.

Visitors take refuge from the heat in the slender, slotted shade of crape myrtles. Maples that might have sprouted large, leafy canopies were planted originally but did not thrive and had to be replaced. So, even in its youth, the memorial continues to evolve.

Maintenance crews pamper the space. A Salvadoran immigrant, Lucas Avilio Guzman, delicately removes gravel from the walkways that is ever finding its way into the shallow pools beneath each bench. He was working in a hotel across the highway from the Pentagon when Flight 77 struck, and he fled the area on foot amid the chaos. He remembers the faces of the survivors he encountered.

“They were nervous wrecks,” he recalls. Now he manicures gravel in the space that honors the survivors. His broom moves like a pendulum; it’s as if he’s tending a giant version of a Zen sand garden, pulling out gravel this week that will be back in the pools the next.

Guzman will arrive early on Sept. 11 to spruce up his Zen garden for the arrival of dignitaries for an invitation-only ceremony commemorating the 10th anniversary of the terrorist attacks.

Heidenberger will wait until the crowd disperses.

He has been to the memorial at many times of day and under many circumstances, often stopping there after his 20-mile weekend bike rides. But he doesn’t like to attend events there on the 11th of September. That’s a day he reserves for moments when well-intentioned speeches don’t drown out the sound of burbling water. When he can be alone on a simple bench. Alone with his memories of Michele.