So here is an interesting observation. Two days ago I cleared the openings to the "pipe tunnel" and within about 2 hours time, both ends are completely backfilled again.
The red line illustrates where the burrows that the monitors dug are, and the circle represents the sizable chamber they have hollowed out down there. Light finger taps on the surface indicate a large hollow area right there. (plus I can hear them moving around down below)
The only reason I can think of why they keep back-filling that pipe, it must be affecting the humidity inside the burrow where they dug.