This paper describes an autonomous platform for exploration and navigation within networks of tunnels, as those typically found in underground mines and caves. In this context, we propose a system allowing two modes of operation: exploration/surveying mode and autonomous navigation mode. In the exploration mode, a remotely located supervisor instructs the platform to move through successive sections of the network, gathering range data that is then concatenated into 2D/3D survey maps of the environment. In the navigation mode, the supervisor specifies high-level missions on the previously acquired survey maps. A motion planner then translates each mission into a set of consecutive navigation actions, separated by natural landmarks. Mission execution consists of autonomously detecting landmarks, self-localizing and performing the planned navigation actions. Advanced and innovative features, mostly related to exploration capabilities, navigation mode switcher, and the integrated aspect of our system distinguish it from the systems described in the literature.