Cyber-Physical Systems (CPSs) are characterised by the interaction of various agents operating under highly changing and, sometimes, unpredictable environmental conditions. It is therefore fundamental to verify whether these systems are robust against perturbations, i.e., whether systems are able to function correctly even in perturbed circumstances. In this paper we present the Software Tool for the Analysis of Robustness in the unKnown environment (Stark), our Java tool for the specification, analysis and testing of robustness properties of CPSs. Stark includes: (i) a specification language for systems behaviour, perturbations, distances on systems behaviours, and properties of those distances; (ii) a module for the simulation of system behaviours and their perturbed versions; (iii) a module for the evaluation of distances between behaviours; (iv) a statistical model checker for formulae in the Robustness Temporal Logic (RobTL), a temporal logic for the specification and verification of properties on the evolution of distances between the behaviours of CPSs, and thus also of robustness properties.