For humanity, it is the earth that is solid, and the air that is permeable; for chthonity, the opposite is true. They wander just beneath our feet, as we do beneath theirs, sometimes even atop one another. Their world is an inversion of our own, one which rests within a sphere atop an endless sky. Our atmosphere is the soil upon which they tread.
They are entirely unaware of our presence; to them, the sound of our footsteps is nothing more than a calming breeze. They have eyes, though they do not see light; rather, they see the complete spectrum of gravity, formed by the subtle tugs of nearby objects on their retinae. Because of this, the core of our planet serves many of the same roles for them as our sun does for us: it is a colossal orb of superheated matter high above which provides heat and sensory information.
Just as the roles of earth and air are reversed in their cosmos, so too are those of fire and water. Flames compose the majority of the anatomical structures beneath their smoky skin, and they find the heat of the inner earth to be pleasant (at least, in moderation). The oceans, on the other hand, are malevolent: tall, titanic masses of death that churn and foam, and extend as far as the eye can see.
Though intelligent, their tribes are not technologically advanced, for there is little in their world which can be used to craft tools. They burrow their way into the hollows of our canyons with their bare hands, or out the sides of our mountains (for the spaces between our mountains are their mountains). From there, they hunt other creatures of the same elemental composition, butcher them for their tender flames, and cook them over an open fountain.
They cannot know whether or not beings like us exist, but still, they wonder about us. They wonder about our mine shafts, and our wells, which to them, are towers which rise and solidify without explanation. And we wonder about them, and the way our buildings moan and shift as the world beneath us moves imperceptibly.