Dropping artificial reef cubes - GIF

[Toolmaker51]

I would love to see how they make those blocks.

Box culverts and vaults are slip forms; and not holes (needing cores) on all six sides. Flash is visible on those closest to camera, indicating at least part of the mold is inside, shut off outside with plates.
Initial guess says two U shaped parts are cemented together, results of a hinged mold from the inside, set inside a box rather like trench shoring, also hinged.