During one of my earlier weekend runs I had seen a pipe in the ground. Big enough to crawl into! But I didn't have a torch on me so I couldn't see if that lead anywhere. So this weekend I went back with a torch, and a camera. And I shone the torch in! It goes at least a few meters. It's difficult to see what happens then. But it is clearly a fairly tight crawl, so that's best done in a full caving suit. Surely not in my running kit! But a caving suit is quite warm. We're on the eve of a heatwave. I think I may wait until early autumn to check this properly!