An air shower simulation package COSMOS was born in 1970’s and has been continuously developed. A recent major update enables particle tracking not only in the atmosphere but also in arbitrary gas, liquid and solid materials by combining with the EPICS detector simulation package. This paper describes the properties of this extended version of COSMOS, namely COSMOS X. Combination of gas, liquid and solid materials in spherical shells with a common center can be defined as environment. Users can also arbitrary define the electric and magnetic fields. These features allow shower simulations even in the soil, concrete, sea and ice. Also simulations at the Sun and the Mars are possible applications. Flexible input/output control since the previous versions of COSMOS, a set of user hook functions, is also available. In the predefined user functions, information of each particle in transportation can be easily accessed and users can extract information from them. General introduction to COSMOS and new functions of COSMOS X together with some interesting application cases will be presented in this paper.