There is a newspaper clipping that reads "Local entrepreneur buys salt mine," "Cave Johnson to bring science, industry to Upper Michigan," establishing the location of the Aperture Science test labs.
Initially the game was going to be a prequel to Portal rather than a sequel. The character Chell would be replaced with a character named Mell and Cave Johnson would appear rather than just be heard in a series of audio recordings and a new style of game play would have been invented with no mention of the portals. Negative feedback from play testers changed development focus from a prequel to making the game a direct sequel.
GLaDOS may be named for real life Dr. Gladys J. Loring who was one of the psychiatrists who evaluated Mercury Astronaut candidates at Wright-Patterson. These men were put through extreme tests that pushed the boundaries of ethical testing.
In some test chambers, there are hidden rooms called the Rat-Man's dens. Each one of his dens has one of the rat man's paintings.
"It ain't over till the fat lady sings" is a well-known phrase, based on the stereotype of opera performances casting larger women singers in major roles. Staying true to the old saying, Portal 2 concludes with an opera song, sung by a uniquely fat turret with a feminine voice.