Distributed physical user interfaces comprise networked sensors, actuators and other devices attached to a variety of computers in different locations. Developing such systems is no easy task. It is hard to track the location and status of component devices, even harder to understand, validate, test and debug how events are transmitted between devices, and hardest yet to see if the overall system behaves correctly. Our Visual Environment Explorer supports developers of these systems by visualizing the location and status of individual and/or aggregate devices. It visualizes the current event flow between devices as they are received and transmitted, as well as the event history. Events are displayable at various levels of detail. The visualization also shows the activity of applications that use these physical devices. The tool is highly interactive: developers can explore system behavior through spatial navigation, zooming, multiple simultaneous views, event filtering, details-on-demand, and time-dependent semantic zooming.