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• Carbon dioxide recharge and limnic eruption processes in deep lakes are reviewed. • Novel system associating pumped-hydro storage and deep lakes degassing is analysed. • The proposed system gives way to sustainable and useful gas removal from deep lakes. • Technical arrangements for pumped-hydro storage at natural deep lakes is provided. • The levelised cost of storage for the proposed system is analysed. Nowadays, pumped-hydro storage is the most established large-scale energy storage technology, but its implementation is seriously constrained by the availability of favourable sites. The geographical symbiosis between mountainous areas and deep lakes offers good prospects for the detection of such sites. In these areas, the generally meromictic structure of such lakes causes carbon dioxide saturation in the deep waters and, in order to mitigate the risk of lethal limnic eruptions as the world has already experienced, the gas-lift technique is being applied although it has a serious environmental drawback. This paper proposes an innovative and sustainable symbiotic match between pumped-hydro energy storage with the ideal deep lake degassing solution, providing removal of toxic gases from deep layers without polluting the surface waters of the lake. Considering the two Cameroonian "killer lakes" Nyos and Monoun taken as cases studies, the feasibility of the proposed system is analysed as well as conditions for participation in the local power market. As a result, the threat posed by these two hazardous water bodies is transformed into an opportunity to store about 756 MWh of electrical energy. In terms of installed capacity, this can achieve as much as 80 MW available for harnessing variable renewable energy and contributing to peak power supply. Finally, thanks to the number and distribution of deep lakes around the world, the proposed system shows capability to significantly contribute to the global energy storage potential in a context marked by an ever increasing need for flexibility in power systems. [ABSTRACT FROM AUTHOR]