Tombstone was a town in the United States of America.
In October 1881, the First Doctor visited Tombstone in search of a dentist, who happened to be the gunfighter and gambler Doc Holliday. He and his companions Steven Taylor and Dodo Chaplet soon became involved in Holliday's feud with Ike Clanton and his brothers Billy and Phineas. (TV: The Gunfighters)
The Torchwood Institute tracked the Doctor to Tombstone after the Gunfight at the OK Corral. (COMIC: The Time Machination)
Behind the scenes[]
Location information
Though it's possible that a real world location doesn't exist in the same geographic space in the Doctor Who universe, such cases are few and far between. Thus, the map at right is probably a good indicator of the DWU location of Tombstone, Arizona.