what do you reckon this was for

so we’re clearing an area behind our house to put a shed that will serve as my wife’s office. in the area we think we’re putting it, there is a curious artifact from the previous owners.

making the shape of a rounded-cornered rectangle (maybe 20x16 ft? haven’t actually measured it) , there’s a small rubber bumper (pictured, maybe 4” across) that fully encloses an area that has maybe 3”deep sandy soil (not at all how the native soil is here), underlaid with some kind of plastic sheeting.

any ideas what the original purpose of this might have been? and do you see any glaring reasons not to dig through it somewhat to make the foundation for our shed? i’ve been thinking it may help keep it drier under the shed than it might otherwise be in our usually damp climate.

Really old and overgrown volleyball court? Base for an above ground swimming pool?

That is strange.

***Edit***
I forgot about your question to dig through it for a foundation. I don't see why not... though I might dig around the edges a bit, just to make sure there are no pipes or other sorts of surprises.

I'd guess it was for a pool or playground area - swing set, that sort of thing. The sand and rubber boundary were pretty common for a long time, for home playgrounds.