The Canyon stretches north to south 20,000km, with a width of 1000km and a depth of 20km.
The Canyon exists on an Earth sized spherical world, but leaving the Canyon is quite impossible, as explained later.
At the center, is a 200km wide 2km deep sea that stretches the length of the Canyon from the frozen south to the frozen north. The Low Sea has a shore line that averages 1km wide before meeting a 2km cliff face. The Low Cliff. As the very center of the Low Sea is the cloud wall. An upwelling of warm fog that turns to clouds, that then move east or west as weather systems and storms.
There are sections of the Low Cliff that have sloughed into the sea, creating the Low Shore, and slopes up to the First Terrace, the Green Hell.
The Low Shore is dotted with cities, but since these cities almost never see the sun through the thick clouds and constant rain, they are not a popular place to live for most sentient races.
The Green Hell stretches 100km on either side of the Low Sea and is generally heavily forested. With Tropical Rainforests, Swamps, and deep dark woodlands further north and south. The weather is usually raining or snowing, depending on latitude. Strange magics cause the plants and beasts of this level to grow much larger than normal. The sun rarely peaks through the cloud cover.
A 4km tall cliff separates the First Terrace from the Second.
The Second Terrace, or the Emerald Hills, is mostly rolling hills and grasslands, with some light forests. It receives less rain than the First Terrace, but enough for crops and temperate forests to flourish. This is the level where most sentient species live.
Looking out from the Emerald Hills over the edge of the cliff, all you see is a rolling sea of clouds.
Moving outward and upward again is the Arid Steppe. This grass and scrubland is almost free of trees, and boasts year round sun.
100km inland and another 4km upward is the beginning of the Frozen wastes. Oddly, there are ancient evergreen forests dotted about.
And the final layer, if it can be called a layer at all, is the Airless Void. Nothing lives here, nothing can live here.
Forgive the rough map, it’s just there to give a sense of structure.
Fun fact, the horizon on Earth is easily calculated as the square root of your height in meters, multiplied by 13.
Standing at the water’s edge of the Low Shore, you’ll never see the Horizon, it’s covered by clouds and rain… But it would be a hair less than 5km for the average sized adult human.
Standing on the edge of the Green Hell terrace and looking out, The Horizon is 161km out, which is not far enough to see the opposite shore line, but the Horizon is again blocked by clouds and rain, particularly the cloud wall at 100km out.
Standing on the edge of the Emerald Hills Terrace, the horizon is roughly 280km out, once again in the Low Sea.
From the Arid Steppe it’s 360km out, once again in that same region of the Low Sea.
From the Frozen Wastes, the Horizon is roughly 430km out, which puts it in the Low Sea.
And Finally, if you could stand on the Airless Wastes, the Horizon would be just over 500km out, nearly at the center of the Low Sea, but still just barely blocked by the Cloud Wall, just like every Terrace below.