To unearth something is to dig it up. You could unearth a coffin, or even a tee shirt buried in the bottom of a drawer.
To put something in the earth is to bury it, to unearth it is to take it back out. When digging in the yard you’ll probably unearth creepy-crawlies and, with luck, a lost treasure will surface. To bring anything hidden “into the light” is to unearth it. You can unearth the secret affairs of your lover, you can unearth the lies of a corrupt government, and you may even unearth a child’s secret candy stash from under the mattress.