SAN FRANCISCO -- Four decades after a group of California high schoolers coined the term 420, one of the holiday's biggest mysteries has been solved.

Every April 20, stoners flock to places like San Francisco's Golden Gate Park or the University of Colorado, Boulder, to celebrate the high holiday. There are many myths about why 420 became the stoner code of choice. Some (falsely) claim it's because there are 420 chemicals in THC, marijuana's psychoactive compound. Other popular origin stories link the date to Adolf Hitler's birthday, the police code for marijuana possession or the date of Bob Marley's death (which was actually May 11).

The real story, however, probably goes back to a group of high school friends in Northern California's Marin County. Of course, nobody can definitively prove they were the first person to use the term 420 to refer to weed. But what these friends can show is almost as good: They have tangible evidence they were using the term earlier than anybody else can show, and they have a plausible story, with evidence behind it, to explain how they coined the term. That doesn't necessarily mean that somebody didn't use it earlier, as some have attempted to claim, but so far no evidence backs up the alternate explanations.

In 1971, a small circle of San Rafael High School students known as the Waldos (named, they say, for the wall where they'd hang out by the school), heard that a Coast Guard service member stationed at nearby Point Reyes had been tending a secret pot crop that he could no longer take care of. Armed with a treasure map drawn by the Coast Guardsman himself, the friends would meet after school -- at 4:20 p.m. -- to try to find the stash.

Week after week, the Waldos would meet at the school's statue of Louis Pasteur, smoke a little pot, hop in Waldo Steve Capper's car and make the 40-minute trip out to Point Reyes. They never did find the crop, but 420-Louis, and then just 420, became the group's codeword for weed, something the friends could say in front of parents or teachers who weren't in the know. The term slowly but surely spread beyond Marin, thanks in part to the Waldos' ties to the Grateful Dead.

Read more on the Waldos and the story behind 420 here.

The Waldos, who revealed their identities to The Huffington Post in 2012, have enjoyed the minor fame that comes with inventing a counterculture holiday. Their story has appeared everywhere from High Times to the New York Times. They sell "420 Waldos" branded clothing on their website. Lagunitas Brewing Company even named a beer after the clique .

But the recent attacks on their story by skeptics or people claiming to have used the term earlier has galvanized members of the group to find a piece of evidence that had long eluded them. If they could find the Coast Guardsman who drafted the original weed treasure map, Capper reasoned, who could doubt them anymore?