Okay, one night some friends and I were exploring the upper reaches of Anzac in Melbourne. It was the time I was a uni student and draining was my thing because it was cool adventurous and cost torch batteries and car fuel. So after exploring the chamber we too the low tunnel and then at the split went right, up the old blue-stone capped canal. We came to the manhole in the street and went to exit. My friend seemed to be having trouble with the manhole so I said "just flip it", which he tried to do. Instead of flipping, we heard this metallic thud. He slid it aside and we saw the underside of a car. Which suddenly started to move! We ducked, but the car somehow avoided driving into the manhole. As the night sky appeared we climbed out of the drain, kicked the manhole cover back in place and then waved at a startled man in the car. We had banged the manhole cover on the underside of his car and he'd driven forward in shock.
A real "ninja turtles" moment.
Then there are the times I went driving in Mini-10th and we encountered some other people exploring it. But I think I've written than one up here.