A true story that happened in my late teens. I had been dating my girlfriend for about 2 years and we were getting into trying new things. One of these things was outdoor sex, lots of it. Mostly in a field near where I lived. On this occassion we were at her village and decided to go for a walk. Part of the walk included going through a disused rail tunnel which to be honest was very dark. We could not help ourselves and about hafway through the tunnel we started kissing and fondling each other. It was not long before I had turned her around and had her hands on the wall and her skirt uplifted. She was so ready by this point that I slid into her from behind and started enjoying sliding in and out. I did try and keep an eye on the tunnel entrances as we were fucking away. Suddenly I could see somebody approaching one end of the tunnel. Walking a dog. I stopped mid fuck, whispered to my girlfriend be quiet. As the person approached we could not tell if it was a man or woman due to it being so dark. We stayed perfectly still, my cock still inside her, the expectation of being caught was quite thrilling. Our hearts were beating faster as the person neared. It was so dark that we could not make out a face, to our dismay the person just carried on walking passed us, we thought we would have been seen, but no, the person, now we could tell it was a woman was talking to her dog (thankfully on its leash) as she carried on past and out the tunnel the other side. We laughed in relief, then I sprang back into action and started sliding in and out again until I spurted my cum inside her. Did the woman see us? We never did find out who she was and we knew everybody in the village so maybe they kept it as their little secret.