Join us for an insightful exploration of the Tethys Platform, a feature-rich tool for developing and deploying web applications for the Earth sciences. Tethys Platform addresses technical barriers by offering a curated suite of free and open-source software (FOSS) accessible through the Tethys SDK - developers can access powerful tools like OpenLayers, CesiumJS, Bokeh, and Plotly for visualization, and GeoServer, THREDDS, and PostGIS for hosting geospatial and gridded datasets. We will explore the versatility of open-source web GIS and how Tethys Platform enables effective communication of scientific information to enhance the decision-making processes. In collaboration with leading regional organizations worldwide, we will showcase several Tethys portals and web applications that utilize satellite data and geospatial technologies effectively for managing water resources, climate risks, and land use. Further, Tethys promotes interoperability and collaborative decision making while meeting diverse organizational requirements. Our talk aims to provide a holistic understanding of how Tethys Platform uses open-source GIS and web applications to significantly augment decision-making capabilities, stimulate innovation, improve data interoperability, and foster collaboration among GIS professionals, researchers, and policymakers. Additionally, we will discuss the challenges of portability of web applications between different organizations' web servers - government organizations often require hosting and managing web applications on their own servers due to political, branding, and security motivations, rather than relying on third-party websites. Our approach ensures that organizations can leverage advanced environmental analysis tools while maintaining sovereignty over their web applications' deployment and management. Tethys Platform’s many capabilities have garnered significant international interest, evidenced by diverse applications developed by users worldwide.