The PetSafe® Microchip Cat Door Tunnel Extension allows the PetSafe® Microchip Cat Door to be installed into thicker doors or walls, adding 2 in. to the overall depth with each tunnel. Adding the tunnel is simple, with no tools required, and each extension can be secured in to place with glue for a flush, precise fit. Multiple tunnels can be used in order to fit the needed door or wall thickness.