I'm thinking Eve is right about our decision to leave the habitat. I think about three huge reasons—the two rhino-bears that wanted to devour us for lunch—and the third one that wants to eat us for supper. This creature, colossal enough, appears smaller than the other two, possibly a juvenile learning to hunt on its own. Still, it's much bigger than an average adult grizzly.

As we sprint through the forest, ducking under branches and cutting around trees to put anything between us and it, the gray-furred bear with rhino horns rumbles to our left, forcing Eve and I to veer right. We dart between a pair of trees, running as fast as we can, Eve ahead of me, me pushing her along to keep us from getting eaten.

I glance back as the rhino-bear gallops after us, glaring through voracious eyes, saliva slobbering from its mouth and gums.

Beyond the creature, the setting sun glows faintly over the brink of the horizon, all but gone. We're traveling northeast, back toward the habitat and the river, but slightly off course. Useless knowledge if we get eaten alive.

Still, we run for our lives.

Me to Eve: "I have an idea!"

"It better be good."

"We need to zig and zag." My voice comes out between labored gasps. "Long zips between trees. Get this thing off balance. Slow it down."

"Good idea!" Eve slashes hard to the right, and I follow suit.

We bowl through the interwoven limbs which scrape at our arms and face. Behind us, the bear changes direction, and takes up pursuit. With it committed to the new path, Eve angles left at ninety degrees. Straight back onto our northeastern heading. The creature chases after us as we make another desperate dash to the right. Then back to the left.

The sudden route changes have the desired effect. The rhino-bear slows down and struggles to make the adjustments to stay on our tails, falling back as we gain ground. But now it's dark. Nighttime in the forest except for a gleam overhead. The moon.

Shafts of light turn the pitch blackness to gray shadows.

The bear grumbles after us as we break into a straightaway, putting more ground between us and it.

As the forest thins out ahead of us, our path broadens into an alley wide enough to run side-by-side. It's also big enough for the vicious predator to reach full speed, which is faster than us. The creature narrows the gap and the closer it gets, the more determined it becomes.

I chance a glance behind me as it emits an intimidating roar. It does it again. With each roar, its head sways sideways and its mouth snaps open, revealing teeth in the shape of ivory spikes.

"We need to zig," I say. "Now!"

I trail in Eve's wake as she dashes between two trees.

The rhino-bear, twenty to thirty feet behind us, lowers its head and charges into the grove, slamming into a thick trunk and obliterating bark and limbs in its way.

Over her shoulder, Eve, to me. "Time to zag!"

We cut back to the left and burst out of the trees into another clearing. The course alternations enable us to lengthen our lead on the juvenile rhino-bear.

Up ahead, a fallen tree lies at the edge of an even bigger clearing.

As we race toward it, I hook my arm around Eve's. "Jump the log."

She has no time to respond, but as we near it, we leap into the air to make the hurtle.

Eve's foot catches the protrusion of a broken off branch, throwing her and me into a headlong fall over the other side of the log. We crash and tumble to a skidding stop. As we struggle to our knees, the rhino-bear sails over us, roaring with frustration. As if in slow motion, the creature's eyes lock on ours as it flies above us. Its blackened lips flap against its gums, air rushing over its mouth.

"Go, go, go." I lift Eve to her feet and guide her back to the fallen tree.

The rhino-bear lands with a crash of earth and paws. Its momentum sends it careening, sliding down the beginning of a steep slope. It digs into the ground and turns back toward us.

As we round the end of the fallen tree, I realize it's hollowed out and big enough for us to squeeze in.

"Inside," I say. "The log... inside."

Eve scrambles into the opening, my head bumping into her backside, my chin taking the brunt of her heels as they kick up in my face, both of us crawling to hide inside the musty and rotten interior of the downed tree. With the rhino-bear roaring and growling on the outside, we reach the center of the enclosed space and huddle tightly together. I can't see Eve, but I feel her breath on my cheeks. Somehow, we got turned around on the inside to face each other.

I struggle to control my panting while Eve sounds like she's about to hyperventilate.

"Shush." I quieten her.

"You shush," she says to me.

But the angry and complaining bear sniffs around outside the log, honing in on our location, probably smelling us as much as hearing our grunts and groans.

It bangs against the outer bark and a massive horn thrusts through the thinning shell of what used to be a tree. I don't know how a tree has had time to fall over and rot out in this new era of Earth's history, but that doesn't matter. What matters is that we don't get impaled by a rhino horn.

The log shakes with ferocity under the onslaught of the juvenile rhino-bear as it snorts and slams its body and snout against the tree, trying to get to us.

Eve screams and so do I.

The three-foot-long primary horn bursts through the log just above our heads.

We scream louder as the creature rips its horn free, bark and wood debris exploding around us and falling down on top of us. Through the opening above our heads, visible against the backdrop of a break in the canopy, a full moon illuminates the rhino-bear as it rises on its hind legs and then comes straight down toward our faces with its enormous paws.

But on the edge of the incline, the creature loses its balance and lands on top of the log, its six-inch claws slicing through the bark, barely missing our heads. As it pulls back to go at us again, it nudges the tree and shoves it forward.

I feel us rolling.

And before I know it, we're leaving the rhino-bear behind as we tumble like a giant spin-wheel.

As we roll, the breach in the log, torn open by the bear, reveals views of the full moon, then darkness, then black dirt mixed with mud and grass. The nauseating revolutions repeat over and over as we spin down the hillside, fortunately devoid of trees.

As the log continues its tumble toward the unknown, each time my body reaches the bottom of the roll, Eve slams against me, blasting air from my lungs. Her elbows jab into my chest, where my heart pounds like it did when the habitat plummeted in a free fall through Earth's atmosphere. Our foreheads smash into each other, disoriented starbursts shooting across my view of the moon overhead.

At our heads and feet, the log crashes into something, the ends demolished into splintered wood debris. The collision shaves the fallen tree down to the length of our bodies, possibly six-feet long, the window-gash in its trunk giving us brief glimpses of our tumble through the forest.

Under the moonlight through a break in the trees and through the hole in the log above our heads, I see the end of our roll racing toward us—a huge rock shaped like a ramp, its upper surface carved and curled up to a sheer point.

"Hold on!" I yell as the battered log with us inside it launches up and out into a veil of darkness.

Eve's screams pierce my ears as I open my eyes at the highest point of our slingshot through the air. As her wails die down and shock sets in, I hear the rushing roar of something I least expected to see again—the southern snaking arm of the Yellowstone River, cast under the shimmering light of the full moon.

Before I brace myself or say anything to Eve, the log crashes into the water with a gigantic splash and sinks beneath the whitewash of raging rapids.

We're engulfed in seconds, once again swimming for our lives.