Here are some other words that could also fit depending on the context:
* Underground: This is a more general term and doesn't necessarily imply being hidden or secret.
* Subsurface: This refers to the area beneath the surface, but it could also apply to water or other materials.
* Buried: This implies that something is intentionally hidden beneath the earth's surface.
* Hidden: This is a broader term that doesn't specifically refer to the earth.