What is this hinged green plastic panelling under my lawn?

I recently moved into a new house and noticed a couple of patches of green plastic under patches of my lawn.

I had assumed they were protecting power wires going I to the summer house, but it’s not in line with the wires at all.

I decided to scrape back the lawn and moss growing on top and it’s a lot larger than I expected and there doesn’t appear to be anything immediately underneath (at least on one end)

Can anyone explain what these are?

Thanks