Seventeen years ago, four flights were hijacked by terrorists in the tragic attacks on Sept. 11, 2001. But only three planes ever reached al-Qaeda’s intended targets.

The passengers of Flight 93, thought to be bound for the nation’s capitol, overtook the cockpit, crashing the plane into an open field in Shanksville, Penn. Washington was spared, but Flight 93’s 40 passengers died as heroes.

This year, a new monument memorializes those passengers and crew members. The Tower of Voices, a 93-foot-tall structure containing 40 wind chimes, is meant to be a “living memorial in sound to remember the forty through their ongoing voices,” writes the National Park Service.

The C-shaped tower—the final installation on the Flight 93 Memorial site, dedicated last Saturday—was designed by Los Angeles-based Paul Murdoch Architects. Murdoch and his team won a design competition in 2005, and have been working with the National Park Service ever since to create a visitor center, monument, and landscaped grounds for the Flight 93 Memorial, writes the Smithsonian.

“We were watching our screens here on the West Coast feeling pretty helpless,” Murdoch told Smithsonian. “And so there was a commitment to try to do something as designers, as architects. We’re fortunate that we’ve had that opportunity.”

Cast from concrete, the tower’s openings and branch connectors are meant to recall the Hemlock trees at the flight’s crash site while also allowing wind to travel through the structure, making music through the 40 aluminum chimes, each five to 10 feet long. The design required the expertise of a musician, a chimes artist, an acoustical engineer, and wind consultants, along with the usual civil engineers, structural engineers, and lighting experts. According to Fox News, the $6-million-dollar project was funded by the National Park Foundation.

The chimes and openings for wind-flow were designed using music theory, the National Park Service reports, so that the sound produced by each individual chime is “musically compatible” with the sounds of the others.

“The intent is to create a set of forty tones (voices) that can connote through consonance the serenity and nobility of the site while also through dissonance recalling the event that consecrated the site,” explains the National Park Service.

Benches face the opening of the tower, allowing guests to sit and reflect, while a winding path through rings of landscaping alludes to sound waves emanating from the chimes.

Since the chimes rely on wind to sound, the experience will be different every visit, depending on the weather.

“The winter wind is quite severe, at 30 to 40 miles per hour from the northwest, and the rest of the year it comes from different directions and lower wind speeds,” Murdoch told Architects + Artisans of the tower’s environment. “[The tower] is meant to be heroic in stature but intimate in personal experience.”

At an observance ceremony located at the memorial’s visitor center this Tuesday, the names of Flight 93’s passengers and crew were read at 10:03 a.m., the moment the plane crashed. President Donald Trump also delivered an address, honoring “a moment when America fought back,” NBC News reports. He called those on Flight 93 “a band of brave patriots.”