An artificial roof unit and method of forming it under water on a base includes locating a tube of a flexible material divided by dividing walls of a flexible material into an array of compartments running the length of the tube, with a first end of the tube on the base and a second end of the tube above the first end of the tube and under water, securing the tube in position on the base using flexible or rigid elements attached to the base; and partially filling some of the compartments with a filler material so that some of the compartments are adjacent two or more other compartments filled with a filler material to support and be supported by the adjacent compartments. The artificial reef unit may be designed to promote an increase in marine life, or to protect a shore line or harbor wall from damage.
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