https://i.imgur.com/v5qIFcn.jpeg
It's a 600 by 700 plastic pallet? 30mm tall. It has connectors so that you can join multiple pieces.
I'm guessing it is used for flooring? Do you know what this is, and used for? And it would be nice if you can tell me where I can get them!
Edit: These are called drainage cells. Thanks guys!
Used under turf or other garden beds on sealed surfaces, like on green roofs. I think they let water flow to an exit to prevent pooling. I had someone in a near factory unit dispose of many of these last year - you're too late for that freebie.