Photograph by Michael Parks.
A bit of industrial stratigraphy. Some theories claim that the fatal shot fired at Kennedy came from the storm drain on Elm Street, at the foot of the grassy knoll, hence the potential importance of the differing street levels between 1963 and the present day in terms of determining whether or not such a shot would even have been possible.
Given that the only exits from the drain are a manhole up to street level and pipes that are 12" and 15" in diameter, it is perhaps an unlikely location for someone who would be looking to get away quickly.