Nyssa’s “resurrected” (well, cured) by the light that started the Universe, and which now threatens to end it—she is placed, mythologically, in the light of the Alpha and the Omega, which could only emanate from the Center. And here we should note the importance of Norse mythology, for the Center of the World Tree, the axis mundi, is the place where Past and Future, Above and Below, come together in the Here and Now. The very notion of such a union of opposites is implicit in the word “Terminus,” as the place-name denoted here functions as both the beginning and the end of “the line.“
As above, so below—this alchemical principle suggests the repetition of certain structures regardless of scale. The Terminus itself is a labyrinth, as is the ship that brings the Lazars, the ductwork underneath that ship, and indeed the TARDIS itself. A labyrinth is distinct from a maze, the latter being generally multicursal, whereas most labyrinths are traditionally unicursal, symbolizing the path to the Center (the seat of divinity) and back out again. As such, labyrinths are generally traversed twice, going in, then out, making them cyclical in nature.
Which brings us to the central conceit of Terminus itself, namely the impossible astronaut that lies at its heart, who supposedly “created” the Universe—but where did this pilot come from? Surely a time traveler came from a Universe, in which case the Universe wasn’t created but rebooted. Which is ultimately a form of Eternal Return. In which case, what happened to the astronaut’s universe? Wasn’t it, too, destroyed, to bring forth the Universe of our own? That seems to be the implication, hence the Doctor’s determination to prevent it from happening again. The eternal return of death and rebirth is indeed the source of suffering, so it seems.
Alchemically, though, this watery substance is what keeps the fire of the Terminus Light at bay, another union of opposites that actually leads to salvation.