A giant zip wire (other wise known as a Tyrolean Crossing or Flying Fox to those Down Under), is basically a big daddy version of the old kids' 'Death Slide', but on steroids. There are a couple of variations on the theme, the most common being a simple high speed traverse, harnessed to a wire over a valley or gorge. The Tyrolean drop involves unclipping the harness and dropping into a river running below.
Note
The Zip Wire is great fun but really needs to be combined with at least one other activity to make a multiactivity day.