Abstract: A novel method of underwater tunnel construction is provided which comprises the employment of a double-layered flexible mold or form and the introduction of both an inert and a reactive fluid into spacings of the mold so as to produce both the shape and the basic structure of the designated tunnel. Upon consolidation of the reactive fluid, the inert fluid will be withdrawn and air is filled in to provide the tunnel spacing for further finishing work and normal function of the tunnel. When completed, the designated tunnel basically consists of a sandwich-like structural body having a least an inner and an outer layer of impermeable fabrics and sandwiched within these fabrics a relatively thick layer of consolidated mass such as concrete which constitutes the main structure or walls of the tunnel. A second layer of consolidated mass may be provided through the use of a third and outer fabric and the corresponding introduction of the reactive fluid into the enclosed spacing thereof.