I knew I was being hunted before I heard the growl. First there had been flashes of gray and black in the trees around me, too fast for me to make out. Then crackling of dried leaves and twigs as those forms came nearer. And that primal, icy feeling on the back of my neck that told me I'd just moved from top of the food chain to prey.

No one was around to help me, either. This was Yellowstone National Park, one of the last great American wildernesses. I hadn't seen another soul since my friends Brandy and Tom abandoned the hike three days ago, and I'd been lost for two days now. A wave of fear rolled over me, making my stomach clench in a nauseating way. Whatever had growled, it wasn't alone.

New growls emerged from behind the trees—low, guttural, and more menacing than a mugger in a dark alley. I flicked my gaze around, trying to hone in on the source, while I drew my backpack off my shoulders. I had a gun in there which I'd brought along in what I'd thought was over-the-top paranoia. Now I wished I'd brought an Uzi and some grenades, too.