Cipher Hunt

Cipherhunt is an alternate reality game about Gravity Falls created by Alex Hirsch. The goal of the game is to find an actual statue of Bill Cipher by retrieving and decoding clues hidden in various locations around the world, such as Russia and Japan. The first one to the statue will receive a prize, though the hunt is mainly about the journey and not the destination.

Rules
According to Alex, the official rules to Cipherhunt are as follows:
 * This is an unofficial fan tribute treasure hunt made just by [Alex Hirsch]! It has no affiliation with any company!
 * No trespassing/vandalism. Everything is in a public place. Be respectful of your surroundings! Don't be a dummy!
 * Be careful - you search at your own risk! Like Pokemon Go, keep an eye out for Gyradoses!
 * If you encounter Bill, whatever you do, don't shake his hand!

The Beginning
Many theorized that there was an actual Bill Cipher statue in the world based off of the end of "Weirdmageddon 3: Take Back The Falls," which contains both a picture of a real Bill statue and a cryptogram hinting at the presence of treasure "deep within the woods" and "beyond the rusty gates."

Alex Hirsch and the team behind Gravity Falls did not talk about the presence of a statue in the months following the end of the show until the silence was broken on July 20, 2016. On that date, Hirsch tweeted "Are you guys ready?" and then proceeded to begin Cipherquest with a tweet containing the words "Let the games begin #FLSKHUKXQW" and the image shown above. The image contained several cryptograms and clues.
 * After shifting the phrase " # FLSKHUKXQW" three letters back, the text reads "#CIPHERHUNT."
 * The text above the Bill statue reads "WKH XUEDQ OHJHQG KDV FRPH WUXH / FLSKHU'V VWDWXH'V FDOOLQJ BRX / WKH VHFUHW PDS LV LQ BRXU KDQG / WR WUDFH WKH FOXHV DFURVV WKH ODQG." After shifting the message three letters back, the text reads "THE URBAN LEGEND HAS COME TRUE / CIPHER'S STATUE'S CALLING YOU / THE SECRET MAP IS IN YOUR HAND / TO TRACE THE CLUES ACROSS THE LAND."
 * The numbers above the Bill statue read "8-15-10-19-5." After putting the message through A1Z26, Atbash and shifting it three letters back, the text reads "PINES."
 * The text below the Bill statue reads "GRQ'W IRUJHW LW'V DOO IRU SOHDVXUH / WKH KXQW LWVHOI'V WKH UHDO WUHDVXUH / EXW D SULCH DZDLWV WKH ILUVW RQH WKHUH / EH VDIH, EH VPDUW, DQG RI FRXUVH EHZDUH." After shifting the message three letters back, the text reads "DON'T FORGET IT'S ALL FOR PLEASURE / THE HUNT ITSELF'S THE REAL TREASURE / BUT A PRIZE AWAITS THE FIRST ONE THERE / BE SAFE, BE SMART, AND OF COURSE BEWARE."

St. Petersburg, Russia
Decoding the red letters placed across both sets of cryptograms creates the word "RUSSIA," hinting at the location of the first clue. The diagram at the middle left of the image was recognized by Russian fans as an architectural plan of the Kazan Cathedral in St. Petersburg. The diagram at the middle right shows exactly where in the Cathedral it can be found. Using this image, the first clue was successfully found. This points to the second clue being located in Japan, which uses yen as currency.
 * The text on the clue decodes to "Finally the hunt can begin / So switch your Rubles out for Yen / Turn left when you're at the shrine's door / When you reach the statue turn left once more / In the leftmost corner in the back / Is the info that you lack / A sword and a crescent mark the clue / Cipher's statue's calling you."

Chiyoda, Tokyo, Japan
The exact location of the second clue is inside the Kanda Myojin Shrine, which Hirsch visited during his trip to Japan. On the back of one of the emas inside the shrine was a scimitar and crescent with an encrypted message.
 * The text on the clue decodes to "Consider in your quest for truth / the hunter of the fountain of youth / 400 before his name is written / Outside the gate is where its hidden / Find what's LOST to pass the test / From a Shrine that's east to a Shrine that's west."

Atlanta, Georgia
The second clue points to the third clue being located inside a Shriner's temple with the address "400 Ponce De Leon Ave NE, Atlanta, GA 30308." A community stream found a missing poster for Waddles in the area, yet it was water damaged and difficult to read. The poster contains a phone number to call if Waddles is found. When called, the number plays a backwards message. This shows that the fourth clue is located in the Ochre Court building of Salve Regina University in Rhode Island.
 * The poster contains some information about Waddles: "NAME: Waddles. SEX: ??? BREED: Pig. WEIGHT: 15 lbs. Last seen sitting around some rocks near a creek. ADDITIONAL INFO: Oink oink oink oink oink oink (etc.)"
 * The poster also contains an encrypted message. When decoded, it reads: "across from the stones of the springs / you'll find some peculiar things / tied to a root is a lone pink key / dig to find what waits for thee." The purpose of this message is to help find the sixth clue.
 * The phone number, when reversed, plays a message from Grunkle Stan while the X-Files theme plays in the background. The call reads: "Hello, it's Grunkle Stan, and I have a riddle for you! What has two legs during the day, four legs during the night, and uh, it's red and white, and, I dunno, I'm no good at these riddles. It's at Ochre Court, all right? The next clue, it's at Ochre Court. It's, uh, it's like, it's a big old building in Rhode Island. And if you go up the stairs, there's gonna be a bunch of pictures of nuns on the wall. You gotta look behind one of the nun pictures. Sister Mary Hilda Miley. Real, real grouch, that nun. Lift up the picture to find a clue behind the nun, all right? That's the whole thing. So, you know, be careful with the picture. Don't break it. I don't want you to get anyone in trouble, all right? You don't want ol' Grunkle Stan going to jail. Anyway, go to the place, look behind the nun, find the thing, and uh, uh, always? Be...mys...look out for mysteries! That it, did, did I do it? All right."

Newport, Rhode Island
The fourth clue was to be located in the Ochre Court building of Salve Regina University in Rhode Island. However, the university accidentally disposed of the clue before Cipherhunt began. Alex Hirsch promised to give a replacement clue at July 21st, 2016, 12:30 PST, and asked people to stay out of the university in the meantime.

GravityFallsCipher has also made a few cryptic tweets about Cipherhunt in the meantime. The fans have taken to returning to Georgia in order to find out if something was added in their absence. At 12:30 PST, Alex Hirsch tweeted Grunkle Stan's phone number (the same number on Waddles' missing poster), and said to leave a message. When called, the number plays a message. This shows that the fourth clue is to be located near the Griffith J. Griffith statue at Griffith Park in Los Angeles.
 * The first clue is "The Nuns have won / But the battle's not lost / We are still far from done / Find the pink key or pay the cost." This is allegedly an old clue people forgot to solve. It relates to the "pink key" discussed on Waddles' missing poster.
 * The second clue is "When one branch closes, another path may still be open. Return to what was and try again while awaiting something new." It was stated in response to someone asking if the first clue was official.
 * The phone number plays a message from Grunkle Stan. The call reads: "A man who's first name is his last, a statue honoring his past. Right behind him by the sign of the park, a golden head shows light in the dark. I dunno what that means. Well, I don't have to go out there, 'n figure this out. Wonder what's on TV."

Griffith Park, Los Angeles
A golden head of Grunkle Stan was found in Los Angeles, which was the fourth clue. The Stan head came with a slip of paper with a code on it, as well as an invisible ink pen, which reveals hidden code.
 * The most accurate translation of the code reads: "50 + 50 THAT'S THE CITY / FROM ONE ANGLE IT'S QUITE PRETTY / YOU'LL FIND A BOW TIE AND ONE EYE / AT HIS HEAD IS A RECTANGLE / STONES BY STAIRS ATOP THE TOP RIGHT ANGLE / THE STONE WITH THE UV SWIRL CAN GLOW / FIND THE BLACK POUCH THAT'S BELOW." 

Century City, Los Angeles
This clue shows that the fifth clue is located in Century City, Los Angeles. A certain park in the city is trimmed into the shape of the Eye of Providence. Jason Ritter, Ariel Hirsch and a small group of fans went down to Century City and successfully found the black pouch, which contained a USB stick with an image of Bill on it. The USB stick contained an audio file that was later uploaded to Soundcloud. The full transcript of the Soundcloud is as follows:

This shows that the sixth clue is located on the California State Summer School of the Arts campus.

CSSSA Campus, Los Angeles
The clue was found in the sublevel of the CSSSA campus in the form of graffiti on a wall. The graffiti shows a crudely drawn Bill Cipher and a series of hex codes that, when decoded, form co-ordinates: 37°49'19"N and 122°13'59"W. These co-ordinates are located in Piedmont, California, the hometown of Dipper, Mabel, and the Hirsch twins.

Piedmont, California
The rhyme on the Waddles missing poster, as well as some guidance from Alex Hirsch, led fans to the Pink Key tied to a stump, in addition to a small chest with a Cryptex locked behind a five-letter keyword. Using the keyword from the original image of Cipherquest, "PINES," the container was opened, revealing the seventh clue and another key. This clue shows that the eighth clue is located at 1825 N Vermont Ave in Los Angeles, and that the provided key has to be used to open PO Box 27128.
 * The clue reads: "LA HAS THE CLUE YOU WANT / 1825 N VERMONT / 27128'S THE BOX FOR THEE / DON'T BE PUZZLED, USE THE KEY."

1825 N Vermont Ave, Los Angeles
After several hours, the person with the key traveled to the post office box and used the key to open it. The eighth clue was a bag containing several thousand jigsaw puzzle pieces. The box also contained an envelope with a drawing of Bill saying "I HOPE YOU LIKE PUZZLES" in code, thirty seven Stan Bucks with the word "FILBRICK" written in invisible ink on their backsides, one Stan Buck with "FILBRICK" in code written on the front side, and some decoy junk mail.

The jigsaw puzzle is still in development. The portions of it that have been assembled point to the completed puzzle being an image of Bill Cipher with a gnome under him and words in Bill Cipher's cryptogram being found below and to the sides of him. From what has been assembled, the puzzle reads "CORNER," "ODNEY," and "I(unknown letter)LA."

A tweet from Alex Hirsch shows a rough idea what the puzzle should look like upon completion, but with a different code that says "this is where the clue will appear" in Bill code with the previously mentioned "corner" remaining because it had already been put together. Eventually, Alex sent two images of different sections of the puzzle and a virtual version of the puzzle was created to get some help.

P rogress on the puzzle eventually reached a point that a fan was able to guess the location of the next clue. It was located on the corner of Rodney and Tillamook Street in Portland, Oregon.

Though the clue was found without completing the puzzle, Alex Hirsch provided incentive to complete both the actual and virtual puzzles by saying that if they were finished, he would release the Gravity Falls pilot and unreleased cut scenes from the show.

Portland, Oregon
Using what was found in the puzzle so far, a fan went to Portland, Oregon and found a lawn gnome at the corner of the above streets. Buried under the lawn gnome was a View-Master and the ninth clue. The View-Master contained slides depicting the area around Confusion Hill in Mendocino County, California. Confusion Hill is a tourist trap similar to the Mystery Shack. One of the slides also contains an image of Snoqualmie Falls, a waterfall known for appearing in the television show Twin Peaks, an inspiration to Gravity Falls. This points to the tenth clue being located at Confusion Hill.
 * The clue says "To the redwoods you must race / If you're Confused you're in the right place / The giftshop is not too far / The password will buy the eyeball jar."

Confusion Hill, California
A fan went to Confusion Hill and successfully brute-forced the passcode to the eyeball jar. It was "FILBRICK," the word in invisible ink on the back of the Stan Bucks packaged with the jigsaw puzzle. Using the code, the fan retrieved the eyeball jar and found the tenth clue inside. Alex Hirsch has stated that fans will get a prize for taking a selfie with the picture of the Gravity Falls crew at Confusion Hill. This points to the eleventh clue being located at  
 * The clue says "WHERE HAS MY GRUNKLE GONE? / TO STANLEY ST. IN OREGON / A TELEPHONE POLE IS WHERE HE'S AT / WHY'D HE HAVE TO BOLT LIKE THAT?"

Trivia

 * Several of the locations for clues have been tied to religion - the cathedral in Russia, the shrines in Japan and Georgia, and the paintings of nuns in Rhode Island.
 * Several of the locations for clues have been tied to Gravity Falls - the Eye of Providence-shaped building in Century City, the post office near the House of Pies (the House of Pies being the place where Alex Hirsch thought of the name "Gravity Falls"), and Confusion Hill, which is a tourist trap similar to the Mystery Shack.
 * The location of the first six clues, when shown together on a map, form the asterism of the Big Dipper.

Official resources

 * Alex Hirsch's Twitter For official updates on the Cipherhunt.
 * GravityFallsCipher's Twitter For official updates on the Cipherhunt.
 * r/gravityfalls megathread For a detailed timeline of every clue and location.