"Oh god, it's so hard to breathe."
The Underworld was split into several regions — regions that did not follow logic at all. There could be a frost region, and then just a few meters away, there will be an infernal region. Of course, the scientists have also hypothesized a reason for that, but the people just chucked it up to magic.
A Desert region, a Frost region, an Infernal region, a Swamp region, a Forest region, a Meadow region, and a Rocky region. These were the main regions of the Underworld, and one might look calmer than the rest, but all were actually equally dangerous. And sometimes, these regions meld together, creating a chaotic ecosystem that results in an even harsher environment.