There's something that goes back to the stories you read about those things that hang out underneath bridges and snatch kids etc. That's the feeling I always get when fishing underneath a road is that there might be something waiting there to inhale my fly and whatever else comes near it, that's why I love fishing for trolls.
The most impressive of these fish to me are always the snarly looking brown trout that fit the bill of your childhood troll figure - mean, fat ugly snout, questionable character and tactics (see trying to wrap you on bridge pilings and or sticks piled up around said pilings). There's been more than a few times where I've ignored my own rules and gone tramping through a seemingly worthless spot underneath or around a bridge and been sadly mistaken to find a big fish hanging out swimming away from me leaving a big wake behind it. Don't let this happen to you, you've been warned.