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If this doesn’t help you, there are plenty of boulders located above the Rave Cave, making that a good place to begin your search.
Unfortunately, there isn’t an exhaustive list of all of the specific locations around the map where you can find these runaway boulders, but your best bet is to search somewhere on high ground until you come across them. There are quite a few of them around the map, so they shouldn’t be too difficult to find.
Source: https://www.distractify.com/p/runaway-boulder-locations-fortnite
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