Weird thing in garden -reposting-

Hello all,

I was trying to out in some garlic in the garden of the house i bought recently, and I hit something.

There is a linen, underneath there is a plastic cover, and underneath a woody thing with some rusty metal on the corners. It looks like a box or a lid of something. I am suspecting a lid to maybe a cistern or a box of sometype. Shall I keep digging? or leave it alone?