I used to love horror movies so I've never really been scared of them, but there was one night...
I was about fifteen years old. We'd been having a late night viewing of Poltergeist (with the house built on the graveyard) at a friend's house when I decided to walk home. It was about 1:30AM on a moonless night. The town I lived in turned the streetlights off at 1AM, so it was pitch black as I walked across the local sports field.
But I wasn't spooked.
I wasn't even halfway across the field when I heard a rumble behind me. It seemed to be getting closer. I picked up my walking pace and started looking over my shoulder, but I couldn't see anything behind me. The rumbling was definitely getting closer. Then I noticed the ground was vibrating. I started jogging, and then running as the invisible thing behind me picked up pace. Soon it was closer behind me - by now a very loud rumble and the ground was shaking. I was running flat out and still couldn't see anything behind me. By the time I reached the end of the sports field I felt that whatever was chasing me was right behind me, so I stopped and spun around. As I did so a dozen huge white vapourous beings shot up from the ground in front of me with a loud "woosh!" sound. My heart stopped.
It was the sprinklers. I'd been running along an underground reticulation pipe as it filled with water.