Abyss

The Abyss is a geologic feature on the planet Exonia. It is an almost perfectly vertical hole 10 km in diameter that extends all the way down to the planet's inner atmospheric cavity. The outer crust is much thinner around the Abyss, making the total depth of the hole 4,822 km, and also squeezing outward the molten inner layer of the outer crust, making the rock surrounding the Abyss solid and stable all the way through. The Abyss's upper opening is dominated by Abyss City, an Exonian metropolis that stretches along the flat land surrounding the Abyss as far as 15 km, as well as along the sides downward as far as 27 km. Transportation within the vertical parts of Abyss City (aka "Abyss City proper") depends on a spiral highway built into the walls of the Abyss that takes advantage of the constant flow of heavy things going downwards (like food) to provide a gravitational assist to pull lighter things going up using something resembling a conveyor belt or an elevator pulley system. There are, of course, other transports within the city, including countless elevators (mostly built into the rock, but a few suspended outside) and several bridges that span geometric chords of the Abyss. No bridges go all the way across the Abyss, because it would be a very costly bridge that would largely be unnecessary, and because the Abyss experiences a good deal of air traffic as well. Flying to the lower parts of the city is the fastest way to get people or sufficiently light goods up and down, and inhabitants of the city begrudge a very popular skydiving market, which has been the location for all skydiving distance records in the past century.

There are many caves and cave systems that open into the Abyss, and the ores in some of the upper caves were a large part of the motivation for establishing Abyss City, and mines have been extended perpendicular to the Abyss ever since. Far below Abyss City, mines are still common but are either remotely operated or are host to their own settlements separate from Abyss City. Besides air transport, these deeper settlements are connected to the rest of the world via a suite of twenty-four elevators extending from the bottom of Abyss City's spiral highway all the way to the bottom opening of the Abyss, making it the longest elevator ever made that is not a space elevator.