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Tunnels represent strong multipath environments for communications, where wireless propagation differs from regular indoor or outdoor scenarios. Moreover, with an adequate operating frequency choice, a tunnel behaves as an oversized waveguide, extending the communication range but affecting the signal with strong fadings. These fadings have been closely studied in the literature in the longitudinal dimension (tunnel length). Nevertheless, they also occur in the cross section, where they have only been analyzed statistically and on a small scale. In this paper, we present the results from an extensive measuring campaign carried out at the Somport tunnel, in Spain. We have adopted the modal theory approach to analyze the large-scale fadings (also known as slow fadings) in the longitudinal, transversal, and vertical dimensions. More specifically, the modes’ geometry allowed for the adequate placement of transmitters to create different periodic fading structures and to virtually divide the tunnel cross section into sectors, where the fadings presented a phase shift of 180°. This study enabled the development of large-scale spatial diversity schemes and navigation techniques to improve communication and localization inside these scenarios. [ABSTRACT FROM AUTHOR]