Forget about gangsters and bank robbers, the most popular document in old FBI case files has to do with UFOs. At the vault," the FBI's digital reading-room, anyone can go online and view the bureau's most notorious cases. "Since we opened the vault, it's been this memo about flying disks or flying saucers, and it relates to an allegation that we heard from a third-hand, saying that the Air Force had found a couple of saucers out in the New Mexico desert," John Fox, FBI historian, said. The memo's all of two paragraphs. Agent Guy Hottel, then head of the FBI's Washington Field Office, writes that an Air Force investigator "...stated that three so-called flying saucers had been recovered in New Mexico. They were described as being circular in shape with raised centers, approximately 50-feet in diameter. ... Each one was occupied by three bodies of human shape but only 3 feet tall, dressed in metallic cloth of a very fine texture. Each body was bandaged in a manner similar to the blackout suits used by speed flyers and test pilots." It's not just the Guy Hottel memo that's a favorite. There are hundreds of other pages of memos and files in the FBI vault - in the "unexplained phenomenon" section, all about alien and UFO sightings - that are more popular online than the FBI's files on Bonnie and Clyde, serial killer Ted Bundy and other famous cases that have become part of FBI lore.