By Adam Fowler
BBC Radio 4's Pleistocene Park
A Russian biologist has been trying to recreate a fully fledged Ice Age eco-system in a remote corner of Siberia, complete, if possible, with woolly mammoths.
From the plane, the landscape was green - thousands of kilometres of seemingly empty tundra, forest and scrubland, punctuated by oxbow lakes, meanders and intricate waterways.
But from the small boat driven by Sergei Zimov along the Kolyma River, everything was blue.
The vast, cloudless sky was almost perfectly reflected in the water, which stretched for several kilometres between either bank and disappeared like a sea ahead.
Sakha - also known as Yakutia - is a huge Russian province in eastern Siberia, a place of large distances, long histories, and big ideas. Sergei Zimov's is one of the biggest.
At the end of the Pleistocene era - 10,000 years ago - woolly mammoths, rhinoceroses and tigers might have watched our progress from the riverside, and herds of horses, bison, musk-ox and Siberian antelope would have roamed the meadows and savannah to either side.
But now all I can see on the banks are dense willow shrubs, and the only predators are shifting clouds of mosquitoes waiting for me to disembark.
Ten thousand years ago, as the climate warmed, the grass gave way to moss and forest, habitats disappeared, and the large mammals went with them.
Or so the theory goes. But Sergei reckons climate had little to do with it and that all these animals would be thriving here now, if it hadn't been for man overhunting them to extinction.
And to prove his case, he is turning 160 sq km of Siberian "desert" back into the teeming wilderness of the late Ice Age, complete with grazing pastures and animals that have not been seen here for millennia.
If we had travelled north we could have reached the Bering Strait and the US in one day by boat. Instead, we are going south, and back in time, to "Pleistocene Park".
"Moose," Sergei shouts, cutting back the outboard and standing up in the boat. We have travelled a few kilometres upriver and two adults - re-introduced to the area as part of the experiment - and a calf watch back, unperturbed.
"Eat! Eat!" Sergei implores. The moose are key to his grand plan. The more they eat, the more the grass will come back, replacing the moss and providing more pasture for his herds.
So far there are fewer than 100 large mammals at work, making progress painfully slow, but Sergei hopes to increase the herds and within five years have 10 times as many, enough he believes to dramatically accelerate their effect on the habitat.
But it has been a wet spring. Much of the new grass in Pleistocene Park is submerged under water and Sergei's grand plan will fail if our own changing climate deprives his animals of food.
Some grassland areas have been flooded since the Spring
But he is optimistic that the reindeer, horses and moose that he has already brought to the park will survive.
"Next year or two: bison from Canada, then musk ox, then eventually we will have 20 mammals per square kilometre," he says.
We are on dry land now - very dry. It is the first time since arriving in this part of Siberia that the sound of my walking has been a rustle rather than a splosh.
Sergei eventually finds a boggy patch and drops to his knees, grabbing great clumps of the wet moss that is so prevalent outside the park.
"See how easy it comes away. It has no roots so the moisture stays. The animals, their hooves, they disturb the moss and let grasses grow instead. The soil dries out, the animals deposit their fertiliser, the grass grows more; and more animals can graze."
Once the population of herbivores is dense enough, the last part of Sergei's plan is to re-introduce the predators. But what will the neighbours think of wolves, bears and Siberian tigers roaming near their land and settlements?
"OK, so one or two people will be killed, like in India," he replies, "but far more will die of alcohol in this place than from tigers."
There is one big flaw, though, and it is woolly with two great tusks. Where is Sergei going to get his mammoths? "Come," he says. "Back in the boat."
Four hours later, we reach a bend in the river where the bank sheds a five-metre thickness of Pleistocene mud and ice every spring.
I am stumbling through quicksand after Sergie who is picking up mammoth bones every three metres or so. He gives me some sludge to hold and tells me it is thawed mammoth dung.
Mammoth remains are uncovered along the river bank each year
This is where he brings Japanese scientists in search of preserved skin and meat, which, one day, might relinquish enough good quality DNA to recreate a mammoth.
Sergei has no real interest in their work, "but if you create a boy mammoth in a university, you need a girl mammoth, and they need somewhere to make baby mammoths. This will be their home."
I returned from Pleistocene Park, exhausted, covered in mosquito bites and smeared with woolly mammoth poo. I thought I was going to see an experiment in the pursuit of academic knowledge about the extinction of mammals, but it was much more than that.
Sergei's grasslands could also have a significant role in slowing global warming. One of the dangers of future climate change is that it could melt the Siberian permafrost, releasing huge quantities of methane, a greenhouse gas far more potent than carbon dioxide, creating yet more global warming.
Grass insulates the permafrost better than mossy wetlands and so would slow the rate of thaw during any global warming that might be coming our way.
It is in all our interests to listen to Sergei Zimov. His grand vision could mean the mammoth will once again roam Siberia, and that humans might just be there to see it.
Pleistocene Park is on Radio 4 at 2100 BST on Monday 2 July then online for seven days at Radio 4's Listen again page.