To me, buried means you use any sort of tool to place the cache other than your hands.
I think I've found 2 caches in my entire career that were "buried". I didn't SBA them though, becuase I'm just a nice guy that way.
Oh, and probably 2 or 3 more in natural depressions or holes covered with rocks or logs. Those were pretty difficult to find too, I might add.
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Just off hand I would think if a Pirate was going to bury treasure, he would do more than cover it with leaves or anything like that. His object is for no one but him to relocate it. I'm thinking he would put it somewhere that's going to take a job to not only find the spot but then work to get it . Unlike Geocaching where you want some one else to find it. Yet, have it more or less put out of sight so a person just wandering by cannot see it, and under normal conditions would stay that way. Nobody is going to carry a shovel are take a backhoe to find your cache. Like the old saying, "It's the hunt not the kill" .