It's a fantasy role-playing game.

DM: You are Tiberius Stormwind, a dragonborn sorcerer . You are the member of the famous group of adventurers. Previously, Some distance down the path, they come across the mangled remains of four duergar, who appear to have been torn apart by a centipede-like monster. Trinket picks up Grog's scent, and they find the tracks of more duergar dragging some manner of stretcher or cart.
DM: Keeping your eyes peeled, you travel for another two and a half hours down this cavern path. It grows tighter at points where you can only really fit three or four individuals through, it's about 20, 25 feet wide, then opens up sporadically into other small pockets, where the natural topography of this portion of this Underdark cavern structure seems to weave and bend. The warmth and that sulfurous smell that was intrinsic to the duergar portion of this cavern that you were so used to, begins to slowly fade and temperature begins to drop. Still uncomfortable, but it's growing more humid as opposed to dry heat.
Player: It's so good.
DM: After another hour of travel, keeping your eyes out, the temperature drops significantly and is actually quite cold. The heat in the volcanic portion of this cavern structure apparently has passed you now, and you've hit an area of the rock where it is a much more chilled, much more ominous--
Player: It's humid still?
DM: Human?
Player: Humid.
DM: Humid? The humidity begins to fade as well. The air becomes a dry cold.
Player: So we're past that.
DM: You eventually using, which by the way the only light source you have going forward is the light from your staff.
Player: Oh, Light!
DM: I assume you've had it up this whole time or you've been walking in the darkness.
Player: Oh, yes, of course.
DM: (chuckles) You continue down and the topography, which was once the black, jagged obsidian flooring of the previous portion of the Underdark, begins to give way to a more natural gray sand-type texture. Your feet sink about a quarter inch or so with each step, and you get about ten, 15 feet before the color of the sand and the texture changes dramatically. It looks almost like a peppering of small whitish-yellow stones and rocks, as well as bits of black and grey. It's like a giant, rocky pit out in front of you.
