What are these crevices below the wing of B-52 bomber?
I think that this crevice which you have noticed is the hinge line for its aileron. This might not actually be an aileron hinge line, as later model B-52s don't actually have ailerons - if this is a later model B-52, then this may just be the remnants of what the ailerons once were.
My answer is based upon this diagram of a B-52D.