THE WILD BOAR is standing 30 or 40 yards away, at the bottom of a grassy bank, staring right at me. Even from this distance, I can see its outrageously long snout, its giant pointed ears, and the spiny bristles along its back. And it’s far bigger than I expected, maybe chest-high to a man. For a moment it seems to consider charging me, then thinks better of it. When it trots away, it moves powerfully, smoothly, on spindly, graceful legs twice as long as a pig’s, and vanishes into the trees.

I climb back into our VW van, tingling all over. This is northern Ukraine’s Chernobyl Exclusion Zone, a huge area, some 60 miles across in places, that’s been off-limits to human habitation since 1986. At the Chernobyl Center, a kind of make­shift reception building in the heart of the old town, I have to hand over a solid 9 inches of local bills—hryvnia, pronounced approximately like the sound of a cardsharp riffling a deck—sign a stack of agreements, compliances, and receipts, and then get checked on an Austin Powers–style Geiger counter made out of chrome. Finally, under the protection of a guide, a driver, and an interpreter, my photographer and I are free to set off into the zone—as long as we do exactly what our guide says.

A handful of dilapidated roads cross the zone, half overgrown with weeds and grasses, and the whole area is littered with pockets of intense radiation, but nature doesn’t seem to mind. The zone is reverting to one big, untamed forest, and it all sounds like a fantastic success story for nature: Remove the humans and the wilderness bounces right back. Lured by tales of mammals unknown in Europe since the Dark Ages, we’re setting out on an atomic safari.

IT WAS SOON after 1 a.m. on the night of April 26, 1986, that one of the world’s nightmare scenarios unfolded. Reactor 4 in the huge Chernobyl power station blew up. The causes are still the subject of debate, but it was some combination of a design flaw involving the control rods that regulate reactor power levels, a poorly trained engineering crew, a test that required a power-down of the reactor, and a dogged old-style Soviet boss who refused to believe anything major could be wrong.

At any rate, it was spectacular. Eight-hundred-pound cubes of lead were tossed around like popcorn. The 1,000-ton sealing cap was blown clear off the reactor. A stream of raspberry-colored light shone up into the night sky—ionized air, so beautiful that inhabitants of the nearby city of Pripyat came out to stare. When it was all over, estimates former deputy chief engineer Grigori Medvedev, the radioactive release was 10 times that of Hiroshima.

Thirty people died on the night of the explosion or soon after. Two days later, a convoy of 1,100 buses shipped out all the inhabitants of Pripyat, turning it into a ghost city overnight.

In the following weeks, bureaucrats in Moscow designated an 1,100-square-mile Exclusion Zone—roughly the size of Yosemite—reasoning that the farther from Chernobyl people were, the better.

Today, around 5,000 people work in the Exclusion Zone, which over the years has grown to an area of 1,660 square miles. For one thing, you can’t just switch off a nuclear power plant. Even decommissioned, it requires maintenance, as does the new nuclear-waste storage facility on site. The workers come in for two-week shifts and receive three times normal pay.

There are also some 300 people living in the zone: villagers who’ve been coming home to their old farming lands since not long after the disaster, and teams of radio­ecologists from around the world who’ve come to study the effects of radioactive fallout on plants and animals. They’ve effectively turned the zone into a giant radiation lab, a place where the animals are mostly undisturbed, living amid a pre-industrial number of humans and a postapocalyptic amount of radioactive strontium and cesium.

On the outside, the fauna seems to be thriving: There have been huge resurgences in the numbers of large mammals, including gray wolves, brown bears, elk, roe deer, and wild boars present in quantities not recorded for more than a century. The question scientists are trying to answer is what’s happening on the inside: in their bones and in their very DNA.

ONCE YOU ENTER the zone, the quiet is a shock. It would be eerie were it not so lovely. The abandoned backstreets of Chernobyl are so overgrown, you can hardly see it’s a town. They’ve turned into dark-green tunnels buzzing with bees, filled with an orchestral score of birdsong, the lanes so narrow that the van pushes aside weeds on both sides as it creeps down them, passing house after house enshrined in forest. Red admirals, peacock butterflies, and some velvety brown lepidoptera are fluttering all over the vegetation. It looks like something out of an old Russian fairy tale.

Ukraine officially opened Chernobyl up to tourism in January 2011, but small groups have been able to visit the zone for the past few years. There are small tour operators based in Kiev that take visitors on day trips. You don’t need Geiger counters or special suits; you just have to stay with the tour, pass through several checkpoints, and get tested for radiation on your way out. The tours will shuttle you around some of the main sites—the deserted city of Pripyat, a small park filled with old Soviet army vehicles used in the cleanup, various concrete memorials to the fire crews who lost their lives after the blast.

The most contaminated of the villages were bulldozed and buried soon after the explosion, with only a few mounds and ridges left to show they were ever there. The meadows are mostly gone, replaced by forest. Russia is a land of forests, but the true forest, the primeval untouched forest that human eyes may never even have seen, is called pushcha—which roughly translates as “dense forest.” This is what has been reestablishing itself at Chernobyl, regenerating at an unprecedented rate.

At the edge of Chernobyl, we stop by the half-mile-wide Pripyat River. It’s unbelievably peaceful. Frogs plop into the water, dragonflies hover—it’s like a weight has been lifted from the world. The main sounds are the different shades of hissing of wind in the trees: high nearby, deeper and steadier farther away. This must be what life was like 1,000 years ago, when the entire human population of the globe was roughly 250 million.

Our tour guide, Sergey, tells us about the herds of boars he has seen, 50 strong, rampaging through the forest. And about a starving wolf pack that surrounded a scientist friend of his in a wood one winter day. He had to shoot every last one to get away.

Is this the world before humanity? Or after? Is there a difference?

TODAY THERE ARE around 5,000 adult wild boars in the Exclusion Zone. In 1995 there were many more, but they suffered an epidemic and have now stabilized. There are 25 to 30 wolf packs, a total of maybe 180 adults. Many more lynx live here than before, along with foxes, barsuks (a Ukrainian badger), hundreds of red deer, and thousands of roe deer and elk. Out of the disaster comes a paradise of wildlife. The Garden of Eden is regenerating.

But it’s not so straightforward.

For 17 years, biologist Igor Chizhevsky has been studying how animals metabolize cesium and strontium. On the surface, Igor says, the wildlife seems to be thriving, but under the fur and hide, the DNA of most species has become unstable. They’ve eaten a lot of food contaminated with cesium and strontium. Even though the animals look fine, there are differences at the chromosomal level in every generation, as yet mostly invisible. But some have started to show: There are bird populations with freakishly high levels of albinism, with 20 percent higher levels of asymmetry in their feathers, and higher cancer rates.

Covered in radioactive particles after the disaster, one large pine forest turned from green to red. Some birch trees have grown in the shape of large, bushy feathers, without a recognizable trunk at all.

“Genomes, er, unpredictable,” says Igor. “Genome not exactly same from generation to generation. They change.”

No one knows what these changes could result in. “Soon or late,” Igor says, “new species will evolve.”

In other words, new animals could actually be in the making here. The area has become a laboratory of microevolution—“very rapid evolution,” says Igor—but no one knows what will emerge or when.

Visiting the Exclusion Zone is like staring down the barrel of our likely fate. We may wipe ourselves out with a nuclear holocaust, or with carbon and methane, or some other way we can’t yet conceive of. Cyano­bacteria poisoned their own atmosphere two and a half billion years ago by releasing vast quantities of a gas that was poisonous to them—oxygen—and in the process created an atmosphere suited to higher forms of land life. Who knows what creatures may adapt to a high-carbon, high-methane atmosphere if we keep going the way we are?

They may include us, or not.

WE DRIVE ON to the old power station itself. It’s a large area of vast concrete buildings. One of them is the stricken Reactor 4, some 200 feet tall, with a giant chimney still rising out of it. For almost 25 years it’s stood encased in a “sarcophagus” of cement, but the seal is far from perfect, and it leaks dangerously.

There are canals threading through the giant buildings, which provided water for the old coolant system, and in one of them the catfish have grown to prodigious sizes. We stop on a metal bridge and gaze down into the brown water. Suddenly the monsters rise to the surface, some of them a good 10 feet long, black, whiskered, curling around as they hunt for the bread people feed them.

They’re not big because of radiation, Sergey insists. It’s just that they haven’t been fished for a quarter of a century.

The whole area is like this: fecund, scary. Later Sergey takes us to an army barracks where some soldier friends of his keep a few wild pets. From the dark doorway of one of the sheds issues a terrific subterranean grunt, and a moment later, as if in a hurry, out trots another wild boar. It comes straight at the fence, presses against it with the weird, wet sucker of its long, long nose, then raises its bristly head and eyeballs me as if I’m something from another planet.

In a pen next door there’s another forest sprite—the barsuk, a very close relative of our badger. When it comes out of its kennel, it runs up a woodpile, turns at the top, and proceeds to stare right into me with deeply strange eyes. Something in me seems to recognize something in it, and I feel a pang of longing. Is it for the deep forest, the pushcha? For the trees, the smell of autumn leaves, of mushrooms and mold? For the freedom to live our own way, far from society?

Crouching and staring, the barsuk doesn’t move a muscle. It could be a stuffed animal, with eyes of glass. Or perhaps a new species, staring at the world with new eyes.

By Henry Shukman. From a longer article that appears in Outside magazine. Used with permission. All rights reserved.