CHERNOBYL, Ukraine — On contaminated land that is too dangerous for human life, the world’s wildest horses roam free.

Across the Chernobyl exclusion zone, Przewalski’s horses — stocky, sand-colored and almost toy-like in appearance — graze in a radioactive landscape larger than Luxembourg.

On April 26, 1986, an explosion at the nuclear power plant in Ukraine sent radiation across Europe and forced the evacuation of entire towns, displacing tens of thousands. It was the worst nuclear disaster in history.

Four decades on, Chernobyl — which is transliterated as “Chornobyl” in Ukraine — remains too dangerous for humans. But the wildlife has moved back in.

Wolves now prowl the vast no-man’s-land spanning Ukraine and Belarus, and brown bears have returned after more than a century. Populations of lynx, moose, red deer and even free-roaming packs of dogs have rebounded.

Przewalski’s horses, native to Mongolia and once on the brink of extinction, were introduced here in 1998 as an experiment.

Known as “takhi” in Mongolia (“spirit”), the horses are distinct from domestic breeds, with 33 pairs of chromosomes compared with 32 in domesticated horses. The modern name comes from the Russian explorer who first formally identified them.

“The fact that Ukraine now has a free-ranging population is something of a small miracle,” said Denys Vyshnevskyi, the zone’s lead nature scientist.

With human pressure gone, parts of the exclusion zone now resemble European landscapes from centuries past, he said, adding: “Nature recovers relatively quickly and effectively.”

The transformation is visible everywhere. Trees pierce abandoned buildings, roads dissolve into forest, and weathered Soviet-era signs stand beside leaning wooden crosses in overgrown cemeteries.

Hidden cameras show the horses adapting in unexpected ways. They seek shelter in crumbling barns and deserted homes, using them to escape harsh weather and insects — even bedding down inside.

The animals live in small social groups — typically one stallion with several mares and their young — alongside separate bands of younger males. Many died after their introduction, but others adapted.

Tracking them takes time. Vyshnevskyi often drives alone for hours, setting motion-sensitive camera traps in camouflaged casings attached to trees.

Despite persistent radiation, scientists have not recorded widespread die-offs, though subtler effects are evident. Some frogs have developed darker skin, and birds in higher-radiation areas are more likely to develop cataracts.

However, new threats have emerged.

Russia’s 2022 invasion brought fighting through the exclusion zone as troops advanced toward Kyiv, digging defenses into contaminated soil. Fires linked to military activity swept through forests.

Harsh wartime winters have also taken a toll. Damage to the power grid left surrounding managed areas without resources, and scientists report increases in fallen trees and dead animals — casualties of both extreme conditions and hastily built fortifications.

“Most forest fires are caused by downed drones,” said Oleksandr Polischuk, who leads a firefighting unit in the zone. “Sometimes we have to travel dozens of kilometers to reach them.”

Fires can send radioactive particles back into the air.

Today, the zone is no longer just an accidental refuge for wildlife. It has become a heavily monitored military corridor, marked by concrete barriers, barbed wire and minefields — a landscape of what some describe as grim beauty.

Personnel rotate in and out to limit radiation exposure. Chernobyl is likely to remain off-limits for generations — too dangerous for people, yet full of life.

“For those of us in conservation and ecology, it’s kind of a wonder,” Vyshnevskyi said. “This land was once heavily used — agriculture, cities, infrastructure. But nature has effectively performed a factory reset.”

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Associated Press writers Dmytro Zhyhinas and Vasilisa Stepanenko contributed to this report.