The title of the mission is a reference to the movie Poltergeist (1982) where one of the characters say "They're here" while commenting on ghosts.

Little Things : They Are Here is the first level to include the Little Calebs, which are a reference to Little Ashes in the movie Army of Darkness (1992).

Innsmouth Plaza : The mall is named after a fictional town in a novel by Howard Phillips Lovecraft, called “The Shadow over Innsmouth” (1931).