November 9th, 1990. The base of the North Tower island, situated in the midstream of the Seine. In the foreground, the piles that are 54 meters deep in the subsoil, equivalent to 18 floors of a building, are pushed into the riverbed by a large machine (seen in the background). Within each tube, the woil will be removed, and then within the space will be lowered metal cages that will from the skeleton around which concrete will be poured. Around these metallic structures there will be smaller tubes, used to guide ultra sound detectors to check if the concrete is evenly spread and has solidified without imperfections. Inch by inch, each section is double checked with a sound detector, looking for cracks or errors. The German engineer estimeed that the security control methods on this job were outstanding.