WP3 will be focused on design and deployment of the experimental cavern laboratory, being the main infrastructure the experimental cavern, which will be built in the Canary Islands (Spain) most likely using an abandoned water gallery. Cavern will be completely sealed to prevent leaks when interior is pressurized. During the detailed design of the experimental cavern lab, it will be decided how to bring the cavern to the desired conditions to proof our concepts, and whether it is more convenient to power the required machinery and equipment connecting them to the electric grid or being assisted with a diesel generator set. In any case, the experimental cavern lab will not be equipped with operating compressors, turbines, reservoirs nor any other relevant equipment & infrastructure, and will never become an energy storage plant, being only used to validate our technology up to TRL 4. Safety & Health & Environmental impact regulations will be strictly pre-assessed and respected.