The origin story of The Joshua Tree’s closing (and most experimental) track has become muddled over the years. Sometimes U2 frontman Bono claims it was inspired by his 1986 trip to El Salvador and Nicaragua. Other times, he says it came as a result of meeting a Chilean artist/activist the same year.

The sad part of the story isn’t that a rock singer would get confused, but why. So many of the countries in Central and South America ravaged by civil war and terrorized by dictators shared a common experience. Young students and dissidents had protested these brutal regimes only to be “disappeared” by squads that would capture these people, torture them, kill them and bury the bodies in unmarked graves in the jungle.

This happened in El Salvador, Chile, Argentina and elsewhere. In each country, the mothers who lost their children became a different sort of protest organization, known as the Mothers of the Disappeared. After learning of these groups, which took roots in the ’70s and ’80s, Bono was inspired to write the song “Mothers of the Disappeared.”

“I have a kind of love-hate relationship with America. I love the place, I love the people. One of the things I hate is that such a trusting people could have put their trust in a guy like Ronald Reagan,” Bono said in a 1987 radio interview. “There is no question in my mind that the people of America, through their taxes, are paying for the equipment that is used to torture people in El Salvador. In my trip… I met with mothers of children who had disappeared. They have never found their children went or where their bodies were buried. They are presumed dead.”

Bono intones, in the opening verse, “Midnight, our sons and daughters / Were cut down and taken from us / Hear their heartbeat / We hear their heartbeat.”

So, as with “Bullet the Blue Sky,” Bono found an angle that applied to The Joshua Tree’s version of Americana. In this closing, hymn-like song, he was depicting what he perceived as the evils of the country’s current foreign policy. While “Bullet” used full-bodied American rock ’n’ roll to combat the dirty wars, “Mothers” went in an experimental direction, devoid of many of the roots-based music on other parts of the LP.

“Consequently, the sound, which was a sound Brian [Eno] came up with for that drum loop that starts off ‘Mothers,’ was very, very evocative of that sinister, death squad darkness,” Adam Clayton said in Classic Albums.

The fuzzy, stark loop was the sound of Larry Mullen Jr. playing a beat on his kit, which was then manipulated by Eno via a tool called the PCM70.

“It’s this box that will resonate certain tones when you choose them. And Brian became a master of using this box,” co-producer Daniel Lanois recalled. “He could actually dial up chord changes as the song went along. But this sort of droning effect very much became the personality of the song.”

Watch U2 Perform "Mothers of the Disappeared' in Chile

The austere and experimental qualities of the song made it a tune that wasn’t the easiest to perform in concert, although U2 included it in a handful of shows on The Joshua Tree tour. The band has since only played “Mothers of the Disappeared” in the name of a cause, even performing it with many of the actual mothers in Chile and Argentina during the band’s tour visits to the South American countries. One of the performances in Chile, in 1998, was also broadcast live to the continent and helped inspire a subsequent protest against former military ruler (and then-current senator) Augusto Pinochet in the country’s parliament.

“So, since then there has been a lot of changes in Chile,” Bono said in a 1998 radio interview, “and it was just nice to feel even a small part of that.”

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