E-Space

E-Space (also known as Exo-Space or Exo-Space-time continuum) was a pocket universe existing alongside its prime universe, N-Space. It possessed an unusual greenish tint. (TV: Full Circle, State of Decay)

Locations in E-Space had negative co-ordinates, where N-Space was positive. When the Doctor's TARDIS materialised on the planet Alzarius after it had travelled into E-space, the TARDIS scanner continued to show images of Gallifrey, which had the positive equivalent of Alzarius' spatial co-ordinates. It was made functional again by replacing its original N-Space image translator with one native to E-Space. (TV: Full Circle)

The planets Alzarius, Terradon, (TV: Full Circle) the vampire planet, (TV: State of Decay) Ballustra, (AUDIO: The Invasion of E-Space) Shapeer, (TV: Warriors' Gate) Isenfel, (AUDIO: Equilibrium) and Apollyon were all located within E-Space. (AUDIO: The Entropy Plague)

Transit between the universes
The Fifth Doctor stated that the negative co-ordinates would make it very difficult for his TARDIS to travel there deliberately and not accidentally. (TV: Earthshock)

By his seventh incarnation the Doctor was able to pilot his TARDIS between E-space and N-space, and was able to retrieve Romana II from the former. (PROSE: Blood Harvest) K9 was also retrieved. (PROSE: Lungbarrow)

CVEs
Travel between N-Space and E-Space could occur via a Charged Vacuum Emboitment, or "CVE". This was a hazardous and apparently random process, even using a TARDIS. (TV: Full Circle)

The Logopolitans created the CVEs which linked together the two universes as part of their plan to forestall the overdue entropic collapse of their universe, N-Space, by opening up what had previously been a closed system and thus dissipating the accumulated entropy.

Following the disastrous consequences of the Master's visit to Logopolis, only one CVE remained open, in the constellation of Cassiopeia (from Earth's perspective). (TV: Logopolis) Unless further CVEs were later created, it was highly likely this was the route through which the Hydrax would later enter E-Space (TV: State of Decay) in the 22nd century. (PROSE: Lucifer Rising)

The Gateway
On his return journey to N-Space the Fourth Doctor and his companions passed through a strange white void known to its most frequent inhabitants, the Tharils, as the Gateway. (TV: Warriors' Gate)

History
The Fourth Doctor, Romana II and K9 Mark II accidentally entered E-Space through a CVE. He landed on Alzarius where he met Adric, an inhabitant of the universe who secreted himself aboard the Doctor's TARDIS to join the Time Lord on his travels. (TV: Full Circle, State of Decay) They spent months trying to find another CVE. During this time, they encountered the Farrian, who had entered E-Space through a CVE. They had opened it themselves using Block Transfer Computation, a skill they learnt from a prisoner. The Farrian attempted to conquer the planet Ballustra, but the Doctor destroyed the device that generated the CVE, forcing them to retreat. (AUDIO: The Invasion of E-Space)

The Doctor and Adric left E-Space through the Gateway. Romana and K9 chose to the stay behind. Romana did so to avoid a return to Gallifrey and K9 accompanied her. (TV: Warriors' Gate)