Unfortunately, the Tombs themselves can't be moved, so if you wish to farm on that particular square of your land, you'll need to first unlock them. There are four different Tombs as of this writing, ranging from small to extra large, and here's a look at the materials you'll need to unlock them.
Small Tomb - 10 Small Picks
Medium Tomb - 22 Stone Picks
Large Tomb - 28 Mallets
Extra Large Tomb - 42 Mallets
As you begin to open these Tombs, you'll win a presumably random prize from the game's pool of possible options. You might win a Guillotine, a Victorian Carriage, a Flying Witch Cat, or something altogether different, with larger prizes obviously being won from larger Tombs and vice versa.
Once you've cleared out your farm's current selection of Tombs, you won't want to delete all of your tools, as expanding your spooky new farm gives you more Tombs to unlock. If you happen to run out of tools, you can also purchase these with Farm Cash, but since there's no time limit on opening them, we'd suggest saving your Cash and going the free route instead.
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What sorts of prizes have you won from your Tombs thus far? Sound off in the comments!