Iodine 129 (129I) is a radionuclide that decays to 129Xe with a half-life of 15.7 million years. The analysis of 129I in the environment has played an important role not only in the fields of planetary science and earth science, but also in the nuclear field in recent years. Particularly, in the case of the Fukushima Dai-ichi Nuclear Power Plant accident in March, 2011, the scientists estimated the distribution and soil deposition of 131I over a wide area, and contributed to the assessment of internal exposure doses in the early stage of the accident. This is one of the most significant achievements in recent 129I-related studies. Future studies are expected to be conducted to elucidate the transport mechanism of 129I from land to river and sea, and to investigate the possible accumulation of 129I in aquatic organisms. The mobility of 129I in the soil studied by the author during the 10-years since the accident has been mainly reviewed in this manuscript.