Inception

[I Took the Red Pill]

ALso I hate to quote myself but I really want to discuss:

How and why did Cobb and Mal go into the first limbo? What were they doing at the time? It only shows them waking up on the floor, there is no one else around that they were trying to extract information from and no one to operate the machine. If there isnt an answer it begs the golden question presented in the film "How did they get there". This seems to support the "the entire movie is a dream" theory

I believe that Cobb, being trained in the field of dream manipulation by his father, suggested that him and Mal share some dreams and explore the endless possibilities (to quote Adriadne: "It's pure creation"). Thing is, either by accident or on purpose, not sure, they ended up going too deep. They live in Limbo for a time until Cobb convinces Mal that they are dreaming, they are killed by the train, and then they wake up. When they wake up on the floor you can see the dream machine next to them. I assume Cobb just pressed the button that lulled them both into the dream state.