For more than 120 years, Fujikura has been manufacturing wires and cables that make up the main artery of energy transmission. In particular, the "undersea cable," a key component in the stable supply of electricity, continues to attract attention as a major project due to its scale, high level of technical difficulty, laying complexity and, above all, its significance as a mission to assist society. Fujikura has also played a leading role in a number of other major projects at home and abroad. Being highly valued for such experience, Fujikura participated in the development project for "KAIKO" ("trench"), an unmanned deep-sea research vehicle that enables underwater depths of 10,000 m (the deepest location on Earth). We were responsible for the development of the primary cable that connects the mother ship and the launcher and the secondary cable that connects the launcher and the child vehicle, and other such cables for marine development. Here, we trace Fujikura's Mariana Trench Roman story, looking at the technological challenges of an unprecedented realm.