Thousands of years in Earth's future, mankind has caused their own downfall. With the rising temperatures and rising sea levels already putting a heavy strain on an unprepared world, it only took a little push to finally spark the trading of nuclear weaponry. The bombs did their work well.
In only a couple of years, the Earth became a shellacked wreck. All the former territorial lines were rendered moot, as there were no longer any proper Countries that could call them their own. Where mankind had grown used to the world seeming smaller and smaller, they were suddenly confronted with the inescapable vastness of it once they no longer had the facilities to produce engines.
Within a few generations, the golden age of Man was little more than a myth, told to the children who hadn't died to radiation poisoning or the attacks of mutated beasts.
However, one thing that in all his fictions and assumptions man had underestimated was the sheer resilience of nature- that, or possibly the ego of man thought that their weapons were indeed capable of ending planets. The scorched forests did as scorched forests have done since time immemorial- they grew.
They grew with a vengeance.
The chunks of road that survived the bombs and warfare cracked and shattered as roots dug their tendrils into concrete. Creeping vines claimed the cities, and soil that had been buried under steel and cement for centuries saw the light of day once again. In a deceptively short amount of time, the Earth returned to a point where an observer could see it and call it blue and green, not smokey and grey.
Still though, despite everything, some remnants of mankind survived.
Mankind are survivors after all; at the end of the day its what they have always done best. They adapted as best they could to this new world, so strange and foreign and yet in some ways reminiscent of where their biological ancestors had evolved millions of years ago. They adapted what tools of the old world they could, and learned anew how to craft what they needed from the land. They no longer built cities as they once had, no longer needing to or living in a world where they even could. However, the corpses of the cities still remained, decaying and crumbling but not gone. For countless tribes of humanity, the overgrown structures were a perfect place to call home.
Meanwhile, back during the war, many countries had made efforts to find a way to survive- some by trying to have a section of land declared a nonmilitarized zone, some by trying to build vast underground bunkers (not one of which survived), and other strategies varying from the pragmatic to the fanciful. None of them worked.
Though one almost did.
A group of scientists had actually been making progress on creating a way to dig a hole through time itself, with the goal to be to simply go forward in time to a point after the conflict had subsided. It was a desperate effort, relying as much on theory as actual fact, but with few other options and some signs of progress they continued despite all safety hazards. Those involved who had family eventually had them live on-site when it got too dangerous outside, with the intent being for everyone to evacuate the present as soon as the project was working.
However, Murphy's law doesn't apply only when it would be amusing. In this case the result was disastrous.
Everyone knew that with the bombs falling, it was a matter of time until the facility was impacted in some regard. Everyone had been hoping that this would take place after they had all left. Sadly, due to random chance, one of the nuclear warheads targeting the region fell directly on top of the facility.
It then failed to detonate.
It didn't need to though, the damage it had done had already sealed the fates of the entire facility. The equipment being used to punch through time itself was highly volatile, dangerous, and jerry-rigged together. The large chunk of radioactive material shearing through countless couplings and doing extensive damage caused a chain reaction that nobody could have predicted.
Creating a feedback loop, the machinery fed off the nuke and in turn began to activate the bomb's detonation sequence. Anyone watching would have reported two explosions, the first being caused by the machinery overloading and the second by the bomb finally going off.
It can't be said that the experiment entirely failed though. The bomb poured enough raw power into the system to supercharge it and bore a hole through time with brute force- the equivalent to using a wrecking ball where the scientists had been trying to use a hammer. Not only did they punch into the future, but they did so with such force that anything too close to the equipment was vaporized (this included the scientific staff, sadly) and anything else in the blast radius (including all the family members and support staff) was pulled into the yawing hole in time. It lasted for only a second, and by the time the nuke went off there was no living thing left in the facility for it to kill.
Those who were pulled through time reached the future, but not the future they expected. It was an overgrown, savage reflection of the world they knew, sparsely inhabited with the kind of humans one would have expected to see existing alongside Tarzan. With no way home and almost no souvenirs from their own time, can they adapt to live in the world that Earth has become?