It has become common knowledge that, when an observer gazes up at the stars from Earth, they see them as they were at some point in the distant past. Furthermore, the further away these stars are, the further back in time they appear from the present. What is not as often discussed, however, is that in this same model of understanding, the observer is looking upon those stars as they were at a time when the universe was significantly smaller. Were anyone to build a telescope which could see the furthest edge of the universe, they would see it as it existed at the moment time began: a moment when it was so small that, paradoxically, its edge was closer to the point occupied by the observer’s eye than their own telescope's lens.
The Tunneler Engine employed by modern spacecraft takes advantage of this anomaly in the properties of space-time. Because the bottom of the universe is never more than a single Planck length away, it is possible, with a sufficiently sharp drill, to pierce this surface and pass "beneath" it. The realm which one then enters, which could just as well be said to be no realm at all, is referred to by those who traverse it as "Underspace."
To travel through Underspace, outside of both space and time, is to travel through a cosmos where every object is entirely unrelated to any other, suspended in an absence of medium. Every last particle of the traveller passing through it, as well as those of the mechanism that allows them to do so, exists in total isolation from any other particle or force whatsoever. As such, it takes an exceptionally sharp drill to pierce the absence of a medium separating them, one which culminates in a tip thinner than any of the particles from which it is composed.
While it is one problem to enter Underspace, it is another altogether to exit. To tunnel upwards from never and nowhere, then arrive at a defined point in space-time, is to experience the entire birth and expansion of the universe around it. Such a thing can be profoundly disorienting for the first-time traveller, and often results in delusions of godhood. This is because, for them, the act of leaving Underspace is visually indistinguishable from the experience of having destroyed, then rebuilt the entire universe with one's self at its center.