screwing into a "sheet" of stud?
we're redoing the entrance/foyer in our condo and planning to put up a small shelf and hooks on the drywall. my studfinder is telling me there's a "sheet" of something behind the drywall. from what i can tell, it spans about 48" horizontally and runs floor to ceiling. no live wires detected. there's also nothing metal in that, or so my trusty earth magnet is suggesting.
so what could this be? and would it be safe for me to drill?