Cleaning out mystery area

I just discovered a mystery (to me) area in my car that is at the bottom of
the footwell where the angled sheet metal is welded on.  I hope this
description makes sense!  When I pulled out the rubber grommet that is on
the under side furthest forward, lots of nasty black dirt came out onto my
drive way.  Since I9m concerned about all this dirt initiating or causing
additional rust, I9d like to clean it out some way.  Is there a good way of
doing this?
Thanks again,
Mike Gagne
74 midge