This is a hole that leads to the tunnel carrying the creek from the dam to the Juniata River.  When I was a kid you could climb down to the pipe that carried the "Little Stream" and then down to a concrete wall.  I guess someone was afraid that somebody would fall in and put up the railing and then decided to put in the grating to cover the opening.  A true "Belt and Suspenders" type of solution to what we never saw as a problem.