If someone unearths facts or evidence about something bad, they discover them with difficulty, usually because they were being kept secret or were being lied about. Researchers have unearthed documents indicating her responsibility for the forced adoption of children = uncover
If you say that someone has unearthed something, you mean that they have found it after it had been hidden or lost for some time. From somewhere, he had unearthed a black silk suit = dig out
If someone unearths something that is buried, they find it by digging in the ground. Fossil hunters have unearthed the bones of an elephant believed to be 500,000 years old