code/modules/awaymissions/cordon.dm

/turf/cordon	Turf type that appears to be a world border, completely impassable and non-interactable to all physical (alive) entities.
/area/misc/cordon	Area used in conjunction with the cordon turf to create a fully functioning world border.
/turf/cordon/secret	This type of cordon will block ghosts from passing through it. Useful for stuff like Away Missions, where you feasibly want to block ghosts from entering to keep a certain map section a secret.