Some interesting reading for a rainy day.

• Ambrose Bierce – A famous Civil War-era writer decides to leave his cushy life to go to Mexico, only to disappear forever.
• Agent 335 – One of the first U.S. spies was a woman, but who?
• The Axeman of New Orleans – This guy won’t stop coming after people with an axe until everybody plays jazz music.
• The Baychimo – When winter strikes, a ship sheds its crew–and then decides to take off without them.
• The Black Dahlia Murder (NEW!) – Who murdered and maimed this woman, and is she the only victim? [suggested by several people]
• Blair Adams (NEW!) – What is this Canadian man running from and did it find him?
  
• Cahokia – A gigantic ancient civilization along the Mississippi is abandoned, but why? [suggested by
  luminescent-wanderings]
  
• The Canneto di Caronia Fires – Mysterious fires keep popping up in this small Sicilian town.
• Çatalhöyük – A 9,500-year-old city in Turkey had everything going for it, and yet it was abandoned.
  
• The Chicago Tylenol Murders – Someone laces Tylenol with poison and descends an entire city into chaos.
• The Dancing Plague – People are stricken with the need to dance, some to their deaths.
  
• D. B. Cooper – An unknown man hijacked a plane, extorted hundreds of thousands, and then parachuted to freedom.
  
• The Dyatlov Pass Incident (tw: photo of a dead body) – Several experienced mountain hikers go into a mountain range in Russia and die of unknown causes. [see also]
• Ebola in Rome – Did an outbreak of ebola strike ancient Athens long before it ever tormented Africa?
  
• Elisa Lam – A woman, seemingly pursued by an unseen foe, disappears, only to be found inside her hotel’s water tower two weeks later.
  
• Erdstall – There are thousands of still-standing, ancient tunnels beneath central Europe, but no one knows what they’re for. [see also]
  
• Genghis Khan’s Tomb – One of the greatest and most successful rulers of all time, but no one can find his final resting place.
  
• The Hinterkaifeck Murders – Unexplained noises, missing house keys, and an entire family found dead in rural Germany.
• The Isabella Stewart Gardener Museum Theft – Some hacks in police uniforms steal a bunch of priceless art, including a Rembrandt.
• The Isdal Woman – What happened to this woman in Norway, who was she, and why were so many steps taken to conceal her identity? [suggested by
  whalesguts]
  
• Japan’s Ghost Ship Problem – Ships from North Korea keep showing up on Japan’s shores… filled with mutilated corpses.
  
• The Jian Seng – A giant ship is found floating with no crew and no one knows where it came from.
  
• Jimmy Hoffa – A teamster with mob ties disappears, theories abound.
• Joseph Newton Chandler III  – Is this dead identity thief the Zodiac Killer? And if so, what is his real name?
  
• The Joyita – Crew members abandon a real unsinkable ship, but why?
  
• The Lighthouse Mystery – Several Scottish lighthouse keepers disappear abruptly.
• The Lost Colony (on Roanoke Island) – A bunch of white people decide to try and colonize an island and it doesn’t go well.
• The Lost Dutchman Mine – In unforgiving territory lies a lost treasure just waiting to be found–if you don’t die first.
• The Lost Nazi Plunder – The nazis stole hoards of important items, including art and cultural artifacts. Where are they now? [see also]
  
• The Mary Celeste – A sailing ship in working order is found, still at sea, without a crew.
  
• Monsieur Chouchani – Who was this mysterious Jewish teacher and mentor of Elie Weisel who dressed like a vagabond?
  
• The Oakville Blobs – Gelatinous blobs of an unknown substance rain from the sky.
  
• The Paris Catacombs – A seemingly infinite series of tunnels filled with bones, artwork, and missing explorers. [see also]
• The Phaistos Disc – A mysterious disc, thousands of years old and covered in strange symbols, is found in Crete.
  
• Rongorongo – An undecifered set of glyphs from Easter Island, possibly a completely independent language.
  
• The Tamam Shud Case (tw: photo of a dead body) – A body shows up on a beach in Australia, and how it got there is wrapped in mystery and lies.
• Tarrare – A man who couldn’t stop eating, anything and everything.
• UVB-76 – A strange radio station in Russia broadcasts a constant buzz, broken only by strange readings of names and numbers.
  
• The Voynich Manuscript – An unbreakable code in an ancient manuscript full of strange drawings–what’s not to like?
• The Woman in the Tree (aka Who Put Bella in the Wych Elm?) – A body turns up stuffed into a tree trunk, becomes a local rallying cry.
  
• The Wow! Signal – A strong radio signal from space still has researchers stunned and baffled.
• The Zodiac Killer (NEW!) – Who is the cryptic killer behind a rash of murders? (Other than Ted Cruz.) [suggested by several people]

(These
are the most reliable unsolved stories I could find. There are many
others that require more sources to be believed or already have very
plausible answers.)

Know a good one that isn’t listed? Let me know!