Covert, cross-border tunnels are a security vulnerability that enables people and contraband to illegally enter the United States. All of these tunnels to-date have been constructed for the purpose of drug smuggling, but they may also be used to support terrorist activity. Past robotic tunnel exploration efforts have had limited success in aiding law enforcement to explore and map the suspect cross-border tunnels. These efforts have made use of adapted explosive ordnance disposal (EOD) or pipe inspection robotic systems that are not ideally suited to the cross-border tunnel environment. The Counter Tunnel project was sponsored by the Office of Secretary of Defense (OSD) Joint Ground Robotics Enterprise (JGRE) to develop a prototype robotic system for counter-tunnel operations, focusing on exploration, mapping, and characterization of tunnels. The purpose of this system is to provide a safe and effective solution for three-dimensional (3D) localization, mapping, and characterization of a tunnel environment. The system is composed of the robotic mobility platform, the mapping sensor payload, and the delivery apparatus. The system is able to deploy and retrieve the robotic mobility platform through a 20-cm-diameter borehole into the tunnel. This requirement posed many challenges in order to design and package the sensor and robotic system to fit through this narrow opening and be able to perform the mission. This paper provides a short description of a few aspects of the Counter Tunnel system such as mobility, perception, and localization, which were developed to meet the unique challenges required to access, explore, and map tunnel environments.