Are We Looking Over or Under The Horizon?

With hopes of maintaining the consistency of this Underground world, I've been spending added time designing the locations that the book carries us through. This has proven to be another challenge in the production of this bug-oriented series. As readers and artists, we are used to and accustomed to cityscapes, forests, and other 'human-scale' scenery. Obviously, insects live in our world, but still a world all their own.
One key step in making the scenery more realistic, and logical was brining the journey underground. Under the surface of the earth, pushing the limits of our perception we begin to find a world of new possibilities. Now I need to understand this world, and what it is made of.
And that calls for an Insect-O-Fact!
Insect-O-Fact #9: Soil Layers
The ground beneath us is composed of many layers of soil referred to as 'horizons'. Each horizon has its own distinct composition of soil, sand, clay and mineral deposits. By understanding these levels, we are given a clearer image, or reference point with which to design the various habitats of the subterranean insects.
Much of the activity in the underground takes place within the first two horizons.

1. Horizon 'O' (Humus): This is the top organic layer, made up of much leaf litter, and debris. many insects feed on decaying organic matter, they may be seen here scavenging. Veritably feeding on trash.
2. Horizon 'A' (Topsoil): Much root growth is exhibited here, and seeds are germinating. The humus is now mixed with mineral particles. The roots may prvide for ample living areas, and may offer dwellings for the higher members of insect colonies. Furthermore, the increase in mineral deposits would provide adequate sustenance for the various fungal gardens cultivated by the insects.
3. --6. Horizons 'E,B,C,R': As the horizons progress deeper and deeper beneath the surface of the earth. The surrounding land transforms from soil, to sand and silt, to clay, to bedrock eventually. It becomes gradually less and less habitable to the point that nothing can survive...or can it??