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 what might have been left behind… by anyone sharing the dream who’s been trapped there before.

”

Limbo, as it is referred to in the dream realm, is an expanse of infinite raw subconscious found deep within the human mind.

Limbo exists as a shared space that is not dreamt by any one of the individuals dreaming together, but is a shared space where any one of the minds sharing the dream can make drastic alterations of any kind without limits or obstacles. Because of the apparent lack of limitations, limbo is a dimension that can allow a dreamer to manifest their deepest desires, but also unwittingly trap themselves in a world where they lose their awareness that the world created is not reality.

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Overview

Traveling to Limbo

Limbo can be entered by traveling deep enough through dream levels (precisely four levels as shown in the Fischer inception job). After Fischer dies in the Mountain Dream (level three), 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 is 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. 

Appearance

During Inception, limbo initially takes on the appearance of an endless ocean, but can be altered to look like anything one desires. Under the influence of Dom and Mal, an enormous and elaborate city was built, though it has been abandoned and is decaying since Cobb's last appearance to the city. Several features drawn from Cobb's and Mal's memories also exist, such as previous homes.

When subjects/dreamers enter limbo through a shared dream, it takes on the appearance of how it last was to those who have previously entered limbo. This is shown when Cobb and Ariadne entered limbo, where it looked identical to Cobb's version of limbo and had no features relating to Ariadne.

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 utopian world together and "feel like gods", with limitless creative potential.

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, seeing Limbo as a paradise. Mal would not accept that she was asleep, so Cobb decided to perform Inception on her, planting the idea in her mind that her world was not real.

Thanks to the Inception, Mal was filled with enough doubt to agree with 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" from the dream she wasn't in. The guilt of knowing that his Inception caused her to kill herself continues to haunt Cobb, manifesting as a shade of Mal that actively tries to get Cobb to commit suicide and "join" her in death.

Time

The time scale in limbo is stretched out to a much higher extent than in regular dream levels due to being buried so deep. Very few people have experimented and gone into limbo due to its risks. As a result of the extreme time dilation in the plane, 20⁰ = 1 second in reality (level 0) is equal to 20⁴ = 160000 seconds in limbo (level 4), so people in limbo can experience a year in under 4 minutes of dreaming.

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, and cause their body within the dream to age beyond recognition. This leads to the fear that if Saito dies in the dream and becomes stuck in Limbo, he would forget his arrangement with Cobb after experiencing up to 183 years in Limbo before returning to reality. Near the end, when Yusuf's van was 10 seconds from hitting the water in the first level, the risk was significantly lower, since the upper bound was lowered to 1 hour * 20³ = 333.3 days.

Leaving Limbo

It is implied that dying in Limbo will allow the dreamer to 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 as reality 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 Fischer 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 remembers that he is dreaming and wakes up alongside Cobb, leaving Limbo once and for all. However, despite missing the kick in level 2 (elevator), and dying in level 1 (drowning) and level 3 (explosion), they awake in reality (level 0). This implies that Saito and Cobb killed themselves in limbo to reenter reality the same way Cobb and Mal did.