The Isabella Stewart Gardner Museum was subject to one of the greatest unsolved art heists in history. Around 1:15 a.m. on March 18, 1990, two white men dressed as Boston police officers gained access to the museum after hours by telling security that they were responding to a disturbance in the area. Once they were let in, the unarmed thieves overpowered the guards, securing them with handcuffs and duct tape in separate areas of the museum's basement. Then they set to work swiping everything they could get their hands on in the next hour:

Vermeer's "The Concert"
Rembrandt's "A Lady And Gentleman In Black"
Rembrandt's "The Storm On The Sea Of Galilee"
Rembrandt's "Self Portrait"
Govaert Flick's "Landscape With An Obelisk"
Degas' "La Sortie Du Pelage"
Degas' "Cortege Aux Environs De Florence"
Degas' "Three Mounted Jockeys"
Degas' "Program For An Artistic Soiree"
Manet's "Chez Tortoni"

...and a Shang Dynasty bronze beaker...

Before leaving, the thieves took the security tapes, but forgot the museum's masterwork, Titian's Rape of Europa. More than ten years later no substantial clues have surfaced other than some photographs of the paintings turned over to police in 1997 by a 38-year-old ex-con and antiques dealer named William P. Youngworth III. He claimed to know where they are, and said he could broker their release in exchange for the $5 million reward the museum offers and the release of his friends from prison.

All told, the value in artwork stolen exceeds $300 million.