Mid-Sized Animals These would be animals like ground squirrels, rats, etc., that can fit through a 6" diameter pipe.  The pipe would be ABS or PVC or another non-biodegradable plastic that is not harmful to animals that may chew on it; each would be buried under the BorderTransportationSystem roadway and center towers.  To help prevent snaking of wire and drugs through the pipes the layout has many curves, angles, and switchbacks.  Vent pipes go vertical from the 6" diameter pipes to the center divider of the BorderTransportationSystem; these will provide air for the animals, and also prevent blower or vacuum devices being used to push/pull illegal drugs through the pipes.  Under the pipe in the middle section would be a drainage pit that would collect any rain or droppings {can be flushed with salt water once per week or month as needed}. Index of ... ABS vs. PVC PVC pros and cons