This study reports on the development of a web-based hydrologic educational system (HydroViz) that supports students' learning in hydrology or related earth science subjects. HydroViz ( http://hydroviz.cilat.org/hydro/ ) is designed as a virtual hydrologic observatory and is based on the integration of field data, remote sensing observations, and computer simulations of hydrologic variables and processes. HydroViz can run on a typical personal computer with Internet access and does not require any specific software package, which makes it easy to utilize. HydroViz employs the free Google Earth browser-based plug-in and its JavaScript application programming interface (API) to enable presentation of geospatial data layers in Google Earth and embed them in web pages that have the same look and feel of Google Earth. The decision to use Google Earth within the HydroViz project was driven by the great deal of geospatial data and visual capabilities it provides for hydrologic educational applications. Besides being freely accessible to a wide user community, Google Earth offers the ability to place and visualize hydrologic technical information on a three-dimensional model of Earth, which facilitates students' interactive and visually supported learning. Within a HydroViz setting, students can use Google Earth navigation capabilities to explore the watershed, either on their own or by using the embedded inquiry-based investigations and the supporting layers of hydrologic information. Cascading style sheets (CSS) and hypertext markup language (HTML) describe the look and formatting of each HydroViz web page. With the aid of Google Earth API, it was also possible to create customized buttons and panels for students to interact with and display the data. HydroViz is populated with several educational modules that range from basic activities (e.g., exploring watershed characteristics) to advanced analysis of field data and simulations. Each module is self-contained where instructions and technical questions are embedded within the same screens that show the watershed and its visual displays. HydroViz has been implemented in several classrooms, and evaluation data showed its potential value as a tool for supporting learning.