re: I found this...
Good question and I wonder how many of these are in total.
From the sequenced Voyager 2 images shown in this page from The Planetary Society, it looks like Hyperion is a floating crater.... an imprint of an impact.
Here's a closer look (Courtesy of Voyager 2 and NSSDC/NASA):
http://nssdc.gsfc.nasa.gov/imgcat/html/object_page/vg2_p23932.html