This layer is just the first couple of hundred meters below the surface.
Here you find most underground resources such as gold, silver, aluminum, iron, coal, uranium, etc.
But here you also find tunnels, underground structures and ancient ruins covered by the ages. Unearthing ruins can either be a huge waste of time and money, but your researchers may also stumble upon something special. Maybe the ruin is actually an old spaceship that crashed long ago. Imagine the technology you could learn from it! At the very least, you'd know for sure that you were not alone in The Universe.