It is a lawn by the Ignalina Nuclear Power Plant, a bigger frame showing two reactor blocks to the north:
And that is the lid of one of its RBMK-1500.
The funny thing is, the pattern is discernible only from above, not on the ground level. It looks as an old original design, and the satellite photos were hard to come by then. Also, it was a place of high security (it still is, even if the plant has been shut down).
I've finally seen it last week (from the outside only, however by walking to it on foot). There was FB post but guess I'll write about that here as well, soon.