[ayWCZ7VT]

Autellus says it is likely that the object is built from double-skinned concrete for the purpose of holding large steel wire mesh structures, expected to disturb the propagation of radar and other signals used by any nearby submarines.

During the decades after the war, the wire mesh structures would have eroded away, leaving only the holes shown on the images provided by Ocean X. I doubt there would be much, if any, erosion occurring at the bottom of the sea floor of the Baltic ocean. The water in the depths of the Baltic is virtually without oxygen...