It can also be under the sand.

I once spent a morning cleaning ~50 plastic bags in the wet sand of the beach at low tide. They were empty shopping bags, but had opened up and filled with sand, so they were basketball-sized and really deep; they required _digging_ every single one to get them out. Next day, 50 more were there. There was no way that many washed up in one night. So I did a little digging with a spade, as deep as I could go in the sand, all the way down to my armpit. And I brought up piece after piece of plastic from the depths, punching through bags on the way down; I don't know how many layers deep. Then I realized these "new" 50 bags had just been there under the surface. The layer of sand made free by yesterday's cleanup was now washed away by the tide to reveal them. That was just the worst feeling, knowing that that beach was basically a 1km-long landfill, riddled with garbage at least a meter deep. An extreme example, but it kind of broke me.