Look for well formed straight line rips that end in swirls. These are usually good rips that back eddy in after the rip rounds a point or where there is a hole or some other structure. Best way to verify there is indeed a hole there is to clink a jig off the bottom and see if you lose contact with the bottom when it gets to the hole. let line out and feel for the bottom and you can get a pretty good idea about where it is and roughly how deep it is.
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