"Into Darkness" is a science-fiction novelette by Australian writer Greg Egan, first published in Asimov's Science Fiction in January 1992.
No reason for its sudden appearance is known, but some assume it to be an experiment by aliens from the future to get into the past with both ends of the wormhole accidentally collapsing towards their common barycenter in spacetime.
Going over "The Intake" forces macroscopic objects like people (with microscopic expections like the flow of blood possible) to only travel further towards "The Core", which allows to leave the wormhole.
The short story follows John Nately, a high-school science teacher, on his eleventh journey from "The Intake" to "The Core", who again risks his life to save that of others.
[1][2] Russell Letson, writing in the Locus Magazine, claims that "the story framework is a tense and effective physical adventure, while at the same time the narrator recognizes the metaphorical properties of the space he is traversing.