"Hangar 18" was the secret storage facility at Wright-Patterson Air Force Base in Dayton, Ohio used by the United States Army Air Forces to hold the Ferengi shuttle "Quark's Treasure," after it crash landed in Roswell, New Mexico in 1947, having traveled back in time via time warp from the year 2372.
Quark, Rom, and Nog, who were in the shuttle when it crashed, were kept in an examination room on the other side of the base, but were able to escape with the assistance of two helpful Humans, retrieve their ship and return to their own time by recreating the time warp. (DS9: "Little Green Men")
"The exterior of Hangar 18 was actually the construction mill building at Paramount Pictures, where sets for Star Trek: Deep Space Nine and Star Trek: Voyager were built. In "Little Green Men", to the left of "Hangar 18", one can see another large building, which is Stage 18, where the interiors for the Starship Defiant were filmed. Down the street, one can glimpse Stages 8 and 9, where the Enterprise-D, Starship Voyager, and Enterprise NX-01 interiors stood." (Star Trek Encyclopedia (4th ed., vol. 1, p. 325))
In UFO conspiracy lore, Hangar 18 was a facility located at Wright-Patterson Air Force Base in Ohio where various recovered alien spacecraft, including the one recovered at Roswell, were stored. This implies that the majority of Quark, Rom, and Nog's stay in 1947 actually took place in Ohio, not Roswell. The Star Trek Encyclopedia (4th ed., vol. 1, p. 325) located "Hangar 18" in Wright Field.