Detroit

Detroit is a city in the U.S. state of Michigan.

Season 5
In a reality constructed by Zachariah to try and convince Dean to become Michael's vessel, Future!Dean tells Dean that Sam went to Detroit alone, gave Lucifer permission to possess him, and the Apocalypse proceeded with the spread of the Croatoan virus and the deaths of millions.

After Dean and Sam later fail in their attempt to kill Lucifer with the Colt, Lucifer asks Sam whether he will agree to be his vessel, and when Sam refuses, he informs Sam that he will say yes to him "in less than six months. And I think it'll happen in Detroit."

Later, when Sam, Dean, Bobby, and Castiel go looking for Lucifer with the plan to use the Horsemen's Rings, Dean recognizes that a sudden temperature drop in Detroit is probably a sign that the devil is there. Sam does agree to become Lucifer's vessel, but it is part of a plan and he eventually regains control and throws himself, Lucifer and Michael into the Cage.