The actual culvert!

I couldn't get anywhere near like touching distance and had to settle for this zoomed-in shot. From memory it's approx. 1.5+ mtrs in diameter, and set approx. 15 degrees from vertical. It's situated about halfway up the bank at the head of the dell, and nobody has any idea why, or what for, and only that it was put there sometime in the latter half of the 1800's. 

It's surrounded by old religious plaques, and in living knowledge, it's never been opened.