CesiumJS is a fantastic library for 3D geospatial visualization on the web, but it can be a huge amount of work to build a full-featured application on top of it. Enter TerriaJS. It builds on top of CesiumJS - and is released under the same open-source license - while adding a responsive UI for exploring a catalog of datasets, charts and graphs, a "splitter" interactive comparison tool, automatic region mapping from CSV files, hooks to geoprocessing services, shareable story building, a 2D fallback based on Leaflet, and much more. In this talk we’ll discover many of the advantages of using TerriaJS while exploring how we used open source and open standards to create the NSW Digital Twin Proof of Concept, a catalog and viewer of 3D datasets integrating live transport data, infrastructure (above and below ground), building information models, and cadastral data. TerriaJS also underpins multiple other major projects developed by CSIRO’s Data61 including Australia’s nationalmap.gov.au and the Global Risk Map - built for the United Nations Principles for Sustainable Insurance Initiative. TerriaJS is widely utilised by independent map publishers with an active chat community on gitter.im and more than 450 GitHub stars.