Mystery Tales issue #40, Atlas comics - issue features one of Spider-Man and Dr. Strange creator Steve Ditko's earliest comics jobs for Atlas - the company that would become Marvel Comics five years later. The issue also contains work by recently deceased Supergirl artist Jim Mooney. The cover art is unsigned, and uncredited in either Overstreet or the eBay auction - I do not recognize the style but it does not appear to be Ditko or Mooney.
The otherwise unexceptional issue was most likely chosen for the cover image, which shows a disturbem man gazing in shock out an airliner window at the sight of a floating city in the clouds above another city. Captions read "what was the secret of the mysterious 'Hidden Land'" and "Does it pay to ignore the 'Voice Of Warning!'" A comment at News@rama suggest that one of the stories in the comic explains how to move an island, but i don't know if that's true or not.
The comic book series (though not that issue) also featured work from an artist named Matt Fox.
Issue 40 was published in 1956, the year Joah Locke was born. The series ran 57 issues, from 1952 (the year Terry O'Quinn was born) to 1957 (the year Buddy Holly released the song "Everyday").
The most recent copy I have of the Overstreet Comics Price Guide (from 2001) states that a mint condition copy of the comic would sell for $150.00. A "good" condition copy for $19.00. A very good condition copy is currently available on eBay and the current bid is $153.49.
Two images of the cover are posted, the second one pops to a much larger image of the copy being sold on ebay.