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S. E. , J. Jä, K. K. , A. K. , E. D. et al., provided local expertise in site selection and interpretation. A.T. measured the gas samples, Author contributions Ü.M. conceived the study and planned the field campaign. J.P. managed the field campaign in most regions. compiled and analysed the data, and created the map in Fig. 1. M.E. and J.T. performed the principal NATURE COMMUNICATIONS | DOI: 10.1038, pp.41467-41485, 2018.