A levvel-bloc.

One of the things that were used for the Afsluitdijk.

 A Levvel-bloc weighs 6,500 kg (6.5 tons) and has two ‘pools’ on top and a rough top layer on which algae and plants can grow. Openings between and in the block allow water to drain efficiently. A groundbreaking innovative design in hydraulic engineering that can withstand all storms