Two Centuries of Oceanographic Data in the Indonesian Seas and Surroundings: Historical Trends, Gaps, and Future Challenges Earth System Science Data (ESSD) The Indonesian Seas and Surroundings (ISS) play a crucial role in global ocean circulation, connecting the Pacific and Indian Oceans within the global thermohaline circulation. This region regulates heat, saltiness, and biogeochemical exchanges, making in-situ measurements essential for understanding climate change and disaster mitigation. This study analyzed over 461,865 oceanographic casts from multiple international repositories, with 360,409 casts or 21.97 % rejection after rigorous quality control. The findings revealed that spatial and temporal data collection was very sparse before the early nineteenth century, with a significant increase in the number of observations beginning in the mid-twentieth century. Data coverage is highly concentrated along international shipping lanes, including the Makassar Strait, Malacca Strait, and South China Seas, while vast areas, such as the Halmahera Sea, Timor Sea, Java Sea, and Sulawesi Sea, remain under-sampled. The most data collected are temperature and temperature. The critical gaps in deep-sea observations, particularly below 800m, where temperature, salinity, and other ocean essential variables data are severely lacking. Furthermore, this finding also highlighted the lack of data in the coastal areas. With the complex features in this region, the existing collection of data introduces uncertainties in Indonesian Throughflow (ITF) variability, ocean heat transport, and monsoon forecasting. To address these gaps, this study proposes deploying autonomous monitoring technologies (Argo floats, gliders, moored buoys) in deep-sea and coastal regions, expanding regional observational networks, and enhancing data-sharing mechanisms.