Uh....nice puddle.
That's because under the sand there's apparently another layer of plastic, and it's been raining. It's not really that big, less than a foot deep.
What are you doing with all that pressure treated lumber?
Giving it away on my local freecycle.org list. This one chick has taken almost all of it, and I seriously wonder if she's got an obsessive disorder and is just piling it all in her backyard, because I just can't see how anyone could reuse it. Seriously, it's all rotted and full of broken-off rusty screws.
Plus, 90% of the boards are under 4 feet because we had to cut up most if it with a circular saw (because of the aforementioned rusty screws all snapping apart instead of politely unscrewing.) She's told me she's building a freaking greenhouse with it, of all things. But whatever--better her backyard than mine.