The hidden passages around the Posse's base that already saved them once and that Carl now wants mapped and defended.
Carl reflects at the council that he still needs to talk to the Changeling about the tunnels around their own base. They have already saved the Posse once, and Rosetta had known about them, but no complete picture exists.
What Carl wants is a map of every tunnel in the area — partly because they could be used for troop movements during the next phase of the war, and partly because anything that useful will also have to be protected before someone else exploits it against them.
A network of concealed underground passages threading the ground around the Posse's base and the surrounding area.
Hidden infrastructure Carl intends to map, exploit for troop movement, and defend.
Also appears in books you've already read — spoiler-safe to their endings: