Cones around the manhole

I saw a square of orange cones today
Yellow caution circling around
a man hole cover.
You know what I thought of?
The teenage mutant ninja turtles.
Futurama and the mutants in the sewers.
The Dark Knight Rises and Bane's underground.
As I passed the display I had to look back
and wonder
what they were keeping
down there
trapped.