We describe our work on the development and use of collaborative virtual environments to support planing, rehearsal, and execution of tactical operations conducted as part of mine countermeasures missions (MCM). Utilizing our VR-based visual analysis tool, Cave5D, we construct interactive virtual environments based on graphical representations of bathymetry/topography, above-surface imags, in-water objects, and environmental conditions. The data sources may include archived data stores and real-time inputs from model simulations or advanced observational platforms. The Cave5D application allows users to view, navigate, and interact with time-varying data in a fully 3D context, thus preserving necessary geospatial relationships crucial for intuitive analysis. Collaborative capabilities have been integrated into Cave5D to enable users at many distributed sites to interact in near real-time with each other and with the data in a many-to-many session. The ability to rapidly configure scenario-based missions in a shared virtual environment has the potential to change the way mission critical information is used by the MCM community.