You could lift it and see whats going on underneath. I found an old brick septic tank under one of our lawns.

I was just going to say exactly the same thing. Mine was full of bottles (nothing rare, unfortunately, but one hell of a lot of whisky and wine was drunk before it was filled), and the top, a slightly domed 'roof' was only a few inches under the grass. I part unearthed the brick culvert that led to it and made a water feature.