The term "in-earths" refers to something that is buried or concealed underground. There are a few different synonyms that can be used to describe something "in-earthed." One common alternative is "buried," which implies that the object or substance has been intentionally hidden or deposited below the surface.
Another synonym for "in-earthed" is "entombed," which has a slightly more dramatic connotation and implies that the object is somewhat trapped or imprisoned underground. A third potential synonym is "subterranean," which is a more general term that can be used to describe anything that exists or occurs underground, rather than specifically referring to something that has been buried or hidden away.