Limbo

Dom and Mal's world built in Limbo, ruined due to being in a state of disuse for so long.

“

Ariadne: Limbo? Arthur: Unconstructed dream space. Ariadne: Well, what the hell is down there? Arthur: Just raw, infinite subconscious. Nothing is down there. Except for whatever that might have been left behind by whoever's sharing the dream who was trapped down there before.

”

Limbo, as it is referred to in the dream realm, is an expanse of infinite raw subconscious.

Limbo exists as a space that is not dreamt by any one individual, but is a shared space where any mind can make drastic alterations of any kind.

Overview

"Death"

It is to be understood that death within a heavily sedated sleep will send the sleeper's mind to Limbo, but as shown by Fischer in the Mountain dream, when one dies and goes to Limbo, the sleeper's 'dead' body actually behaves more like a sleeping body. Eames is able to administer a kick to Fischer in the form of the defibrillator, that, when synchronized with a kick from within Limbo, can wake him up back in the Mountain Dream where he "died". 

Traveling to Limbo

Limbo can be entered by traveling deeper through the dream levels. After Fischer dies in the Mountain Dream, Cobb and Ariadne are able to enter Limbo by sleeping. The only reason this is possible is because, as Yusuf establishes earlier, the team cannot create more than three stable levels of dreams. Therefore, sleeping in Level 3 results in a sleeper dreaming themselves into Limbo. It should also be noted that manually entering Limbo allows a sleeper to retain the knowledge that they are dreaming, making it easier to avoid becoming 'lost' in Limbo. 

Dom and Mal's Experiments

Dom and Mal's experiments in Limbo

At some unspecified point in time before the Fischer Inception Job began, Cobb and Mal were performing experiments to see how deep they could travel within their minds. Eventually they manually dreamt themselves into Limbo. Here they were able to create a perfect environment together and "feel like gods".

Eventually, the couple began to build from their memories, making the distinction between reality and dream more difficult to distinguish. Over time, Cobb became dissatisfied with Limbo because he knew that it was not real, while Mal was beginning to accept it as her reality. Mal would not accept that she was asleep so Cobb was forced to perform Inception on her, planting the idea in her mind that her world was not real.

At this point Mal was filled with enough doubt to allow her and Cobb to kill themselves on train tracks in order to wake up. After waking, the idea Cobb planted in her mind was still growing and Mal still believed she was dreaming. Eventually, she committed suicide in an attempt to "wake up". The guilt of knowing that his Inception caused her to kill herself continues to haunt Cobb. 

Time

The time scale in limbo is stretched out to a much higher extent than in dreams since it is not a dream level, but a separate plane. Very few people have experimented and gone into limbo due to its risks. As a result of the extreme time dilation in the plane, people in limbo can experience years in a minimal amount of time. The resulting effect on the mental state can be difficult to overcome upon waking, as the years experienced in limbo can result in the dreamer believing that what they have experienced is actual reality.

Getting Out of Limbo

It is implied that dying in Limbo will allow the dreamer awaken in reality again. However, because of the extreme time dilation in Limbo, the dreamer is unlikely to realize that the world the dreamer is experiencing is actually a dream. As a result, the dreamer will perceive the dream world is real and will not be able to wake from Limbo.

Cobb and Mal manage to leave Limbo by committing suicide, presumably straight into the waking world. Robert Fischer and Ariadne also manage to leave Limbo by falling from a high balcony, kicking them back to the third dream level of the Inception dream. During the Inception job, Saito is unable to immediately leave Limbo as a result of unconsciously falling into Limbo and experiences countless years in Limbo, perceiving it to be reality. When Cobb finally appears before him in Limbo, Saito and Cobb remember that they are dreaming and wake themselves up.