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The Comprehensive Nuclear-Test-Ban Treaty (CTBT) bans nuclear explosions anywhere on the Earth surface, in the atmosphere, underwater and underground. The treaty has a unique and comprehensive verification regime to ensure that no nuclear explosion goes undetected, supported by the International Monitoring System (IMS). The IMS has established observatories around the world using four monitoring technologies: 1) seismic, 2) hydroacoustic, 3) infrasound, and 4) radionuclide. These monitoring data are used not only to detect nuclear explosions but also for civil and scientific purposes, including earthquakes, volcanic eruptions, tsunamis, and dispersion of radionuclide materials. This session aims to discuss the technologies employed in the CTBT IMS and their applications in earth science. This session also covers research related to radioactive fallout from past atomic bombs. As time passes, observing fallout becomes increasingly difficult. To more accurately assess the scope of such analysis, we invite research contributions that enhance our understanding in the development and simulation of numerical models, and field surveys to obtain verification data.