Can you imagine how a today city could have been many centuries ago, say, in the Renaissance, or in the Baroque period? Did you ever desire to explore that city not only as it is now, but also as it was at those times? And wouldn't you like to get explanations of what you are seeing while you proceed in your exploration? We are developing a web system that allows you to meet these desiderata in a virtual world. Our system allows the developer to integrate a number of 3D virtual reconstructions of the urban structures of a city (e.g., streets, squares, buildings, etc.), each one representing the city in a different period of time. End users can not only navigate each virtual space, e.g., "walking" along a street or getting into a building of the virtual town in a given period, but can also select a different historical period and switch to the representation of the same urban context in that time period. In this way, users can experiment a form of navigation which exploits two dimensions of virtuality: the virtual space, reconstructed via 3Dgraphics, and the virtual time, which is achieved by the possibility of switching among different temporal frameworks. In addition, at any point of space and time, the user can get various kinds of information concerning what (s)he is currently exploring, by jumping into a "traditional" 2D information space made of textual or multimedia web pages. From a technological perspective, the system uses:

• VRML for implementing 3D representations of urban structures, where users visualize themselves as avatars in the virtual worlds;
• World-view as VRML viewer;
• Java for loading of dynamically created virtual contexts
• XML for the representation of Web pages linked to the VRML virtual worlds

The final paper will present the motivations of the project, will discuss the various modes of user interaction and navigation available in the system ("walk", where the user perceives him/herself as virtual actor embedded in the virtual space, and "fly", where (s)he has a panoramic, "from-the-sky" view of the space context) , and will describe technical aspects of the system. The approach will be exemplified by presenting a case study, based on the 3D-multitemporal reconstruction of Milan along various periods, from the 15th century to today.