The sheet metal acts like a faraday cage and basically absorbs any wifi signals, preventing them from getting inside. It's the same reason your phone reception drops to almost zero when you're inside an elevator. The only time you really need a trench between two buildings is if they don't have line of sight with each other. If they do have LOS, all you need is some way to carry the signal past the sheet metal. The remaining connection can be can be done over wifi via LOS.
If there are holes in the sheet metal, like windows, you can set up the antennas there to try extend the signal. i.e. Router at window in building 1 facing window in building 2. Wireless repeater in window in building 2.
If there are no windows, but there's an opening big enough to run a cable from inside to outside the buildings, you can get something like this:
http://www.amazon.com/Amped-Wireless-Outdoor-Directional-AD14EX/dp/B004WPUHXO
And either set up a wireless bridge or a repeater between the two buildings.