This was one of my old childhood stamping (no pun) grounds!
The tunnel drains water from what were marshes (now ponds)behind the road, though I never figured whether it served any other purpose. It's a long way from Pennance Mill and even further from Swanpool, so not connected with either of those. It might just have been a speculative trial, like several others in the area, or maybe it really was to drain the marshes though seems an expensive way to have done it!