Charged Vacuum Emboitement
A Charged Vacuum Emboitment or CVE served as a transit point between the main universe, N-Space and the smaller universe, E-Space. The Logopolitans created the CVE's using Block Transfer Computations to save N-Space, their universe, the existence of which entropy threatened.
While all complex systems eventually break down, the Logopolitans reasoned that if they opened up a closed system (N-Space) in a smaller one (E-Space) they could extend the life of the larger one. By the year 1981, N-Space would have already past the point of no return and the main universe would have "died" without the CVE's keeping it still viable. CVE's operated passively, drawing physical objects through it as well as entropy. Because of their special function, the closure of the CVE's, then, threatending the physical integrity of N-Space and all life there. (DW: Logopolis, NA: Timewyrm: Apocalypse).
While in flight, the Doctor's TARDIS travelled through a CVE into E-Space by accident (DW: Full Circle). A similar event may have happened to the Human ship Hydrax which passed into E-Space (DW: State of Decay) in the middle part of the 22nd century (NA: Lucifer Rising).