They heard the waterfall before they saw it; a dull roaring, like the sound of thunderous rain heard from inside a greenhouse, just audible over the roar of the tender's engine. Ahead of them, visible above the forested hills that lined the river, rose a white cloud, stretching to the east as it was driven by the wind. The crew of the tender glanced at each other nervously as the small boat continued to power its way up the river.

They turned a bend, and there it was ahead of them; a horseshoe of white, falling down a sheer cliff where the land rose precipitously ahead of them. The base of the waterfall was hidden from sight by the white mist thrown up by the continuous, crashing impact of water, and enough of it carried on the wind to wet their skins and dampen their clothes. Fielding took the tender over to the shore while the three security men, their guns in their hands, scanned the water around them for any trace of hostile wildlife.

"Looks like this is as far as we go," said White, his eyes on the trees that stood a few yards back from the sandy beach, alert for the first hint of movement. He had to raise his voice to be heard above the waterfall. "We going back now?"

"This could be why we've seen no sign of human life so far," said Fielding. "You normally find towns and cities on rivers because they're like natural roads. You can sail boats up and down them, but a waterfall stops that. It'd be a great tourist attraction, though. I bet there's a visitor's centre up there, at the top."

"Can't see anything," said Willard, shading his eyes with his hand as he stared upwards.

"I think I saw a way up there," said Fielding, though. "There was an animal track going up through a gully, back there a bit. A bit of a hike, but I think two of us could make it."

"Two of us?" asked Conrad.

"Meaning no offense, but it might be a bit of an effort for you two, er, older folks," said Fielding apologetically. "Roy and James'll stay with you, keep you safe, while me and White go see what's up there."

"We're a bit more spry than we look," said Agnes, though. "If you can make it up there, we can as well."

"We often go for a hike in the hills," added Conrad as the second officer looked at them doubtfully. "It keeps us young and healthy, and we'd also like to see what's up there."

Agnes nodded, a smile on her face, and Fielding nodded back. "If you're sure you're up to it..." He said.

"We'll let you know if we get tired," said Conrad. "With all our hiking experience keeping us fit and healthy, though, we'll probably beat you up there."

"Okay," said Fielding, "but you keep to the middle of the group. I want men with guns ahead of and behind you. God knows what wildlife there is up there. Jaguars, wolves, bush dogs..."

"A T-rex perhaps," said White with a mischievous grin.

"Pelagornis and Paleomastodon are both cenozoic creatures," pointed out Conrad.

"I have no idea what that means," White replied.

"It means we won't come across any T-rexes," said the paleontologist confidently. "Or any other non-avian dinosaurs. The creatures we've seen so far come from a time long after the dinosaurs became extinct. We don't have to worry about coming across any of them."

"We hope," his wife added, though. "Until we find out how there are Pelagornis and Paleomastodons here, we can't say for certain what we'll find."

"A hidden, secret laboratory maybe," said Roy Ellis. "Where scientists are bringing extinct creatures back to life."

"If it's hidden and secret," said Fielding, "letting the birds fly around loose is a bit of a giveaway."

"An abandoned laboratory, then," said Ellis. "They made a mistake, the animals got loose and all the scientists were eaten."

"Let's not get carried away with flights of fancy," warned Fielding. "Instead of guessing, let's go find out."

Fielding turned the boat around and took them a short distance back down the estuary, to where the trees grew almost all the way to the water's edge. They found a big one, and White waded ashore to tie the mooring rope to it. Then they all jumped down into the knee-high water and splashed their way onto dry land, after which they took their shoes off to wring the water out of their socks.

"It's about midday," said Fielding as he put his shoes back on. "We need to get back here in three or four hours if we're to get back to the ship before dark. We walk for an hour, then we turn around and head back. That gives us time to solve any problems we might come across."

"Understood," said Willard. "Let's go, then. Make the best use of the time."

Fielding took the lead, setting off up the steep slope with White just behind. They both had guns in their hands, and kept a wary eye out for anything that might be dangerous. Conrad and Agnes followed, and Willard and Ellis brought up the rear, also with their guns in their hands. They followed an animal trail that seemed to have been made by something about the size of a large dog. To their left, the waterfall continued to thunder, sending a fine spray of mist to wet their clothes and stick their hair to their faces.

"How come there isn't a proper set of stairs?" muttered White to himself. "I mean, whoever owns this place, the government or some billionaire, must occasionally want to go down to the water's edge. How does he get there?"

"There probably is a set of stairs somewhere," Ellis replied. "Just not here. It might even be on the other side of the river."

"No, this is the best way to the top of the hills," the other man told him. "We got a good look at the hills from the boat, remember? Every other way up is steeper. Back home, all the oldest roads started out as animal paths. The people who came to live there widened them, smoothed them, put in stairs where necessary. The same thing should have happened here. Why didn't it?"

"You actually think we've travelled back in time?" asked Ellis. "Of all the possible answers to this mystery, that's the craziest."

"Is it?" asked White. "Think back to the last time you were on a beach. There was litter, right? No matter how far you were from civilisation, there were bits of plastic washed up on the sand. Bits of polystyrene. Bits of rope. Stuff thrown away by some careless idiot maybe halfway around the world and drifting for however many years until it washes up somewhere. The environmentalists go crazy about it. Did you see any litter back there? Any of you? Even a little bit?"

"Maybe the ocean currents just don't bring litter this way," Ellis suggested, "and there's no-one living upstream."

"Right," said White, "but how can that be? A river that wide must flow for hundreds of miles. How can there be no-one living along it in this day and age? No logging camps. No mining companies. No farms. How is that possible?"

"Maybe it's a nature reserve."

"Then there would have been a time before it was a nature reserve. Plastics last forever. There'd be something." He took a deep breath. "I'm telling you, man. This place freaks me out."

"Yeah," Ellis agreed. He was silent for a couple of minutes before speaking again, as the six of them trudged their way up the steep, treacherous slope. "Yeah. Me too."

They trudged onwards up the steep slope, through tangling vines that caught at their legs and over bare rocks that shifted treacherously under their feet. Fielding looked back from time to time to see how the Bellamys were doing, but Conrad and Agnes both smiled cheerfully back at him and kept climbing. Fielding sighed doubtfully, but decided they were the best judges of their physical ability. He returned his attention to the way forward, therefore, and kept climbing.

Eventually the slope began to level out, and they saw a vast, level plain stretching ahead of them. Their view of it was blocked by a small copse of trees at first, but they moved past it, Conrad and Agnes panting from the effort, and then they all gasped in wonder at what they saw.

The plain was covered by an ocean of waving, waist-high grass that stretched to the horizon ahead of them, although they saw some hills to the south. The huge river flowed across it, disappearing from view to their left where it plunged into the waterfall. Standing at its bank were a herd of creatures that looked like nothing they'd ever seen before. They looked like hornless rhinos, and although it was hard to judge their size from this distance, they looked to be larger than the largest elephant. They were grazing on the reeds that lined the river, lifting their heads as they chewed as if to search for predators, although what kind of creature they had to fear, they couldn't imagine.

"Paraceratherium?" said Conrad breathlessly.

"Not big enough," his wife replied. "Paracera..."

"Not big enough?" cried David White in disbelief. "Look at those things! They're huge!"

"Paraceratherium was twice the size of those things," Agnes told him. "A smaller species, perhaps. I'd need to see a skeleton."

They all took their phones from their pockets and started video-recording the huge creatures.

"Another prehistoric creature?" asked Fielding, staring at his phone screen.

"Yes," said Conrad. "And you can see that they're alert for danger. We need to be careful."

"Anything big enough to threaten them, we'd see it in this grass, wouldn't we?" said Roy Ellis. "We'd see it coming a mile off."

Conrad nodded thoughtfully. There were trees, scattered here and there across the plain, but not enough for large creatures to hide in. So what were the rhino-creatures afraid of? "When did Titanoboa live?" he asked his wife.

"The paleocene," Agnes replied. "Millions of years before creatures like these existed."

"Titano..." began Fielding uncertainly.

"The biggest snake ever," Conrad told him. "Fifty feet long. Three feet wide at the widest part. But don't worry. All the creatures we've seen so far come from more or less the same point in history. Titanoboa came way before that."

"The fossils we've discovered so far came from way before that," his wife corrected him. She turned to Fielding. "The fossil record is like a net. It's mostly made of gaps."

"So there could be anything out there," the Second Officer said, scanning his eyes qcross the vista in front of him.

"Within reason," Conrad replied.

"Shit!" said James Willard with feeling. "Where the hell are we? We're back in time, aren't we? We really are back in time."

"We don't know that..." Fielding began.

"Don't we?" shouted Willard, his voice rising hysterically. He pointed to the huge rhino-creatures. "How long does it take for something like that to grow to full size? If it was created in a secret laboratory, someone would have leaked it to the press. Some disgruntled employee, maybe. Or some explorer or environmentalist would have sneaked in and taken pictures. How can you create something like them and keep it secret?"

"Yes," said Fielding thoughtfully. "That was the part of Jurassic Park that never made sense to me."

"So we're back in time, aren't we?"

"There are other possible explanations that make much more sense," said Conrad.

"Oh yeah?" said Willard. "Like what?"

"Just because I can't think of them right now doesn't mean they don't exist," the paleontologist told him. "Time travel is impossible, though. Forget about it."

"How can you look at those things and forget about it?"

Willard was sounding panicked, and Fielding went to face him. "Whatever's happened, wherever we are, we'll deal with it," he told the frightened man. "We'll figure it out and find our way home." He turned to Conrad. "Right?"

"Erm..." said White, pointing nervously. "One of them's coming in our direction."

Everyone looked, and saw that one of the rhino-creatures was indeed ambling towards them, its eyes fixed on the six humans. "No need to panic," said Conrad, more to himself than the others. "I think it's just curious. It's wondering what we are."

"It's curious because it's never seen a human being before," said Willard, his eyes wide with fear.

"We don't know that," Conrad replied.

"Are we in danger?" asked Fielding?"

"I don't think so," Conrad replied. "Ever been in a field full of cows? They're curious creatures, and will wander over to get a closer look at you. That's all this is."

"I've hears of people being killed by cows," said Ellis.

"If they're startled," Conrad told him. "So nobody do anything to startle it, okay? If things do start to look dicey, we can always retreat back down the animal path. I doubt it could follow us down there."

"We're staying here, then?" said his wife.

"Why not?" said Conrad with a grin. "Don't you want a closer look at that thing?"

She answered his grin with one of her own. "You bet your life I do."

The huge animal strolled towards them at a slow, easy gait, followed by the others, perhaps wondering what their leader had seen. As it grew close, the six humans were awed to see just how huge it was. Its head towered above them, twice as high above the ground as the head of the tallest man. Its legs alone were taller than any of them, and it's body could have contained the body of a fully grown bull elephant while barely having to bend its knees. The humans drew back apprehensively as the creature drew close, and as it lowered its head to peer at them its sweet, spicy breath washed over them with the fragrance of crushed herbs. Fielding chuckled nervously as it nuzzled at his chest with its broad nose, smelling him, and then the others were gathering around, also staring and smelling.

One of them bumped Ellis with a leathery knee, and White steadied him as he stumbled back. Another stretched out its neck towards Conrad, and he reached out to touch it under the chin. Its skin was surprisingly smooth there, and as he stroked it, the creature stretched out its neck like a cat, its eyes closing, clearly enjoying the attention.

"I think we should leave," said Fielding. "These things could crush us and not even know they're doing it. It's about time we were leaving anyway."

"Yeah," Willard agreed, already stepping back, looking behind himself to find the path they'd come up by. "God, they're so big! If one of them got itself riled up, we'd be in big trouble,"

"So let's get out of here while they're still friendly," said Fielding. "The same order we came up by. Let's go."

The huge rhino-creatures watched as the six humans picked their way slowly and carefully back down the narrow animal track. Then they lost interest and strolled back to the river.