In downtown Oklahoma City is one of the enduring icons of the city’s history.

It isn’t a building, or a street. It’s called the Survivor Tree, and over the past 21 years it has represented the best of the city after one of the biggest tragedies in U.S. history.

The 21st anniversary of the Oklahoma City bombing is on Tuesday. To commemorate it, Onside Stories on Monday released a short documentary named “The Tifo,” which tells the story of the commemorative Survivor Tree tifo created by OKC Energy FC Supporters group The Grid last year.

The idea came from a regular annual project assigned by teacher John Bratt of Dove Science Academy, also a member of The Grid back in 2014.

“Every year, they design an Oklahoma City flag,” Bratt tells the documentary. “One of my students, Shae Hale, came up with the idea of the Survivor Tree with two bands of color.”

“Everyone knows what the three symbolizes, and what it’s from, what it’s been through and what we’ve been through, so it brings the whole community together,” Hale said.

“I looked at that and said, ‘That’s one of the better designs I’ve ever seen,’” Bratt said. “I asked her, ‘Hey, can I use this next year on a bigger display for the supporters club, the soccer fans I’m with?’”

With help from both supporters and in particular the school’s students, a massive version of the flag was created. The day before the 20th anniversary of the tragedy, at Energy FC’s home opener at Taft Stadium, it and the words “The Spirit of This City Will Not Be Defeated” were on display for all to see.

“I looked over to the stands when we walked out of the locker room right before kickoff, and that’s when I saw the tifo, and that’s when we kinda looked around at each and said, ‘Hey, this isn’t just for our team right now,” said Energy FC captain Michael Thomas. “Yes, we want to go out and play well, but this is for this whole city. Look how much support we have right now, and look how much this means for them.”

Just over a week ago on April 9, when Energy FC hosted Saint Louis FC, the Survivor Tree tifo was inducted into the Oklahoma City National Memorial’s archive, a piece of living history that symbolizes what the tree means to the city, and its people.

“After the bombing, there was an American Elm that survived when everything else was shattered,” Mike Turpen, Chairman of the Oklahoma City National Memorial Foundation tells the documentary. “And look what happened. It became the symbolism, the analogy – if you will – of survivability, rebirth and the future, and as the tree has grown, so has the city.”