No one knows why they exist, but theories range from serial killers to fertility rituals.

Humans have a natural desire to leave their mark on the world, as either an expression of ego or simple mischievousness.

Most of the time, these markings are pretty ordinary — crass messages written in Sharpie on bathroom stalls, for example, or spray-painted tags on highway underpasses.

But sometimes, the inscrutable symbols and signs that people leave in public places are oddly beautiful. Consider the locks people put on that bridge in Paris, for instance, or the decades’ worth of messages visitors have written to Elvis on the gates of Graceland.

A stranger manifestation of this singular human desire are “shoe trees” — trees in seemingly random places that are festooned with sneakers, boots, slippers, sandals, ice skates and other kinds of footwear.

Unlike the Love Locks bridge in Paris or the gates of Graceland, though, shoe trees exist in hundreds of places, all around the world, and anybody can start one.

But their origin story is much more interesting and complex.

The phenomenon began in North America — at least as far back as the Great Depression, when people living in the same community sometimes hung extra pairs of shoes on trees for others to take, since not everyone could afford to have their own pair.

Woodstock, Illinois. | via Waymarking.com

When the Depression ended, the tradition continued. America became embroiled in a series of wars — first World War II, then the invasions of Korea and Vietnam — and soldiers returning home from battlefields abroad started tying their military boots together and tossing them into trees as a way of saying, “I’m done; time for a new chapter.”

This unusual custom somehow, at some point, spilled out of North America and onto other continents. But no one seems to know how or when or why.

A 60-year-old shoe tree in northeast Idaho that mysteriously burned down in 2010. | via sondahl.com

According to hundreds of users’ photos posted on Waymarking.com, a public platform where people record and categorize unique locations around the world, there are shoe trees in Hawaii, Australia, Germany, the Netherlands, the United Kingdom, South Africa and beyond.

If you read other languages besides English, you’ll find evidence of shoe trees in<