Geologic mapping in urban areas poses a conundrum: little bare ground is available for inspection, but subsurface data, primarily in the form of geotechnical explorations, are abundant. Most of these data, however, are widely scattered and poorly organized in building and utility departments, transportation agencies, and private consulting firms. The Seattle-Area Geologic Mapping Project has developed, and is now populating, a GIS-based relational database to efficiently store, manipulate, and display the vast amount of existing subsurface geologic data across the Seattle area to enhance and facilitate the creation of geologic maps. The database is designed to accommodate geologic data from a variety of sources and formats, to create a common interface for entering and displaying data, and to support current and future scientific and engineering studies. Geologic information from tens of thousands of field explorations, exposures, and excavations have been entered into the database and include such information as location, geologic layer type and depth, and material density and type. The database contains "raw" data as well as fields for geologic interpretation and for the metadata on original source documents, original scale, and data quality. The data, currently stored in ArcView shapefile format and an Access database, are entered and accessed through customized ArcView interfaces. The database is now being converted to an ArcSDE geodatabase with an Oracle database backend. Customized tools have been developed to facilitate the visualization and interpretation of raw data by interactively viewing the subsurface geologic data through a series of cross sections, stick logs, and fence diagrams at user-selected locations. Partnerships have been formed with a number of local public agencies (such as building departments, public utilities, and transportation agencies) both to acquire the raw data from geologic and geotechnical studies and to return the populated database and GIS interface to those agencies. The information will then be readily available to engineers, planners, and the public, identifying locations where non-proprietary geologic data can be reviewed to improve subsequent investigations in the vicinity of existing studies.