I know this is a super long shot, but it's been bugging me. It's a sci-fi short story, and I'm pretty sure it's part of an anthology. All I remember is that it's a pair of people trying to hunt/chase another pair of people through caverns/a fortress, looking for a macguffin of some type. Ring any bells? I don't read a lot of anthologies/compendiums, so it may have been part of Year's Best Sci-fi by Dozois or (less likely) a compendium of shorts by a single author. EDIT: Found it on my Kindle by scanning every short story I have...it's Inevitable by Sean Williams. Whew!