The beginning of the book establishes the framework in which a 17th-century gentleman, mourning the death of his beloved, Lady Mirdath, is given a vision of a far-distant future where their souls will be re-united, and sees the world of that time through the eyes of a future incarnation. The language and style used are intended to resemble that of the 17th century, though the prose has features characteristic of no period whatsoever: the almost-complete lack of dialogue and proper names, for example. Critic Ian Bell has suggested that John Milton's epic poem "Paradise Lost" (1667) is probably a partial literary inspiration for Hodgson's novel, especially due to the hellish visions of sombre intensity which mark both works, and other similarities including the use of massive structures (the Temple of Pandemonium in Milton and the Last Redoubt in The Night Land).
Once into the book, the 17th century framing is mostly inconsequential. Instead, the story focuses on the future. The Sun has gone out and the Earth is lit only by the glow of residual vulcanism. The last few millions of the human race are gathered together in a gigantic metal pyramid, nearly eight miles high â the Last Redoubt, under siege from unknown forces and Powers outside in the dark. These are held back by a shield known as the "air clog", powered from a subterranean energy source called the "Earth Current". For millennia, vast living shapesâthe Watchersâhave waited in the darkness near the pyramid. It is thought they are waiting for the inevitable time when the Circle's power finally weakens and dies. Other living things have been seen in the darkness beyond, some of unknown origins, and others that may once have been human.
To leave the protection of the Circle means almost certain death, or worse an ultimate destruction of the soul. As the story commences, the narrator establishes mind contact with an inhabitant of another, forgotten Lesser Redoubt. First one expedition sets off to succour the inhabitants of the Lesser Redoubt, whose own Earth Current has been exhausted, only to meet with disaster. After that, the narrator sets off alone into the darkness to find the girl he has made contact with, knowing now that she is the reincarnation of his past love.
At the conclusion of the adventure, the narrative does not return to the framework story, instead ending with the happy homecoming of the couple and his inauguration into the ranks of their most honoured heroes.

Question: What lights the Earth in the future parts of the story?
Answer: Residual vulcanism, the Sun is gone. 
Alternative answer: remaining glow of volcanos .