Although web-based 3D virtual simulation environments are appealing applications, their construction in large scale is a challenging task. Existing systems that support 3D virtual environments focus typically on specific tasks and have a supporting structure, which is closely tight to the application. As a consequence, a change in the application usually means a change in the supporting structure too, making it difficult and expensive to develop new or to extend/change existing VE applications. Thus, solutions that can decrease the development time of 3D virtual environments are most welcome. This paper introduces a novel approach for building and extending virtual environments. An XML-based mark up language, VEML, is presented, which describes simulations through the concept of atomic simulation. VEML can be used to create and/or to extend highly dynamic 3D virtual environments, such as marketing campaigns for virtual shopping mall, customized training environments, etc.