A cool example of how nature takes back what rightfully belongs to it is Tikal, one of the most famous remnants of the Maya civilization. When author and journalist Alan Weisman hiked through the surrounding region, he discovered something fascinating on his way: "You're walking through this really dense rainforest, and you're walking over hills," Weisman told Live Science. "And the archaeologists are explaining to you that what you're really walking over are pyramids and cities that haven't been excavated."

We know about sites like Tikal because humans have gone to great efforts to dig up and restore their remains. Meanwhile, countless other ruins remain hidden, sealed beneath forest and dirt. "It's just amazingly thrilling how fast nature can bury us," Weisman said. 

This scene from Guatemala's rainforest allows us a glimpse of what our planet could look like if humans were no more. Lately, have been getting snippets of this idea, as the global COVID-19 pandemic has kept people inside, encouraging animals to return to our quieter urban environments. Weisman, who wrote The World Without Us, spent several years interviewing experts and systematically developing scenarios that would unfold in our planet if we disappeared.

In his research, Weisman started by taking a look at cities, where some of the most dramatic and immediate changes would occur, thanks to a sudden lack of human maintenance. Without people to run pumps that divert rainfall and rising groundwater, the subways of huge sprawling cities like London and New York would flood within hours of our disappearance. "[Engineers] have told me that it would take about 36 hours for the subways to flood completely," he said. 

Without human oversight, glitches in oil refineries and nuclear plants would go unchecked, likely resulting in massive fires, nuclear explosions, and devastating nuclear fallout. "There's going to be a gush of radiation if suddenly we disappear. And that's a real wildcard, it's almost impossible to predict what that's going to do," Weisman explained.

In the wake of our demise, we would also leave behind mountains of waste — much of it plastic, which would likely persist for thousands of years, with huge effects on wildlife.

Persistent organic pollutants (POPs) — human-made chemicals such as PCBs that currently can’t be broken down in nature — would also leave a mark. "Some of these POPs may be around until the end of time on Earth. In time, however, they will be safely buried away." The combined rapid and slow release of all the polluting waste we leave behind would undoubtedly have damaging effects on surrounding habitats and wildlife, however, that doesn't necessarily mean total destruction: one quick look at the rebounding of wildlife at the site of the Chernobyl nuclear disaster shows that nature can be resilient on short timescales, even under such extremes.

Meanwhile, petroleum waste that spills or seeps into the ground at industrial sites and factories would be broken down and reused by microbes and plants in just a few decades. 

With all that polluting legacy unfolding, water running underground in cities would corrode the metal structures that hold up the streets above subterranean transport systems, and whole avenues would collapse, transforming suddenly into mid-city rivers, Weisman said.

Over successive winters, without our regular de-icing, pavements would crack and provide new niches for seeds to take root — carried on the wind and excreted by overflying birds — and develop into trees that continue the gradual dismemberment of pavements and roads. The same would happen to bridges. Add some general degradation and these structures could collapse within a few hundred years. 

With all this fresh new habitat opening up, nature would march in, pasting over the formerly concrete jungle with grasslands, shrubbery, and dense stands of trees. Weisman said that would cause the accumulation of dry organic material, such as leaves and twigs. "Fires are going to create a lot of charred material that will fall to the street, which is going to be terrific for nurturing biological life. The streets will convert to little grasslands and forests growing up within 500 years," he said.

According to Weisman, buildings would degrade from hundreds of years of damage from erosion and fire. The first to topple would be modern glass and metal structures that would shatter and rust.

On the other hand, "buildings that will last the longest are the ones made out of the Earth itself" — like stone structures, Weisman added. But even those would ultimately become a softened version of themselves: the defined, iconic skylines we know so well today would disappear.

Earth might eventually become lusher and more diverse — but we should also address climate change, arguably humanity's most indelible impact on the planet. Weisman said that making useful predictions about what will unfold can be tricky. For example, if there are explosions at industrial plants, or oil or gas wellheads that continue to burn long after we're all gone, huge amounts of heat-trapping carbon dioxide would continue to be discharged into the atmosphere.

But carbon dioxide doesn't stay suspended in the atmosphere forever: our oceans are essential to absorbing vast amounts of carbon dioxide from the air. Of course, there are still limits to how much of it the ocean can take up without its own waters acidifying to unhealthy levels — potentially to the detriment of thousands of marine species.