A whole lot has changed since the first Earth Day in 1970.

Back then, the most urgent environmental problems facing the United States were air and water pollution. In the decades since, we've made huge progress mopping that up, only to discover entirely new headaches, like global warming and ocean acidification.

Even today, our understanding of the Earth itself — and the ways we're transforming it — keeps evolving with each passing year. We've uncovered entirely new geological features and ecosystems. We've brought endangered species back from the edge of extinction. We've altered the atmosphere, both for better and worse.

Here's a list of some of the most surprising, encouraging, and worrisome things we've learned about Earth just over the past year:

1) Scientists discovered thousands of new mountains — on the ocean floor

Nothing illustrates how much we still have to learn about our home planet like the fact that we're still discovering new mountains. Thousands of them, in fact.

"Most of the places ... have never been mapped before"

In August 2014, scientists were mapping a largely unexplored swath of ocean floor near the Johnson Atoll in the Pacific Ocean when they suddenly stumbled on an entire mountain, some 3,300 feet high, that no one had ever seen before (shown above). "These seamounts are very common, but we don't know about them because most of the places that we go out and map have never been mapped before," explained James Gardner, who led the mapping effort.

That was just the beginning. A few months later, a team of researchers announced they'd identified another 15,000 new seamounts — in addition to the 5,000 or so already been discovered. The scientists used satellite measurements and gravity modeling to publish the most detailed maps ever of the ocean floor.

2) Scientists also discovered a few bizarre new species

At this point, scientists have described about 1.5 million different species on the Earth. That sounds like a lot, yes, but estimates suggest there are another 4 million species still waiting to be discovered.

Every year churns up more surprises. Sometimes these discoveries are incremental. Last year, for instance, scientists announced that the Araguaian river dolphin in Brazil was actually a distinct species from the well-known Amazon river dolphin.

But other times, researchers come across something truly unexpected. In December 2014, scientists exploring the Mariana Trench found what they believed to be a brand new species of snailfish living 26,715 feet below sea level. That's the deepest fish ever recorded. (See video above.)

We also discovered a new stick insect in Vietnam (the second-longest insect ever seen), a Moroccan flic-flac spider with a bizarre method of leaping about, a new species of wild banana in India, and two new species of venomous jellyfish off western Australia's central coast. See here for a longer list.

3 ) But we also learned we've wiped out 50% of wildlife since 1970

Of course, at the same we're discovering new species we're also imperiling existing ones. And we got a stark reminder of that in September 2014.

But we've also managed to help some species rebound

A major study by the World Wildlife Fund estimated that the overall number of mammals, birds, reptiles, amphibians and fish declined 52 percent between 1970 and 2010 — which was far more than anyone realized. (Note: that doesn't mean we've wiped out half of all species. It means that, on average, the world's vertebrate species populations are about half the size they were in 1970.)

The main culprits? Humans, who have been wiping out other animals through hunting, fishing, deforestation, pollution, and various forms of habitat destruction. Freshwater species were suffering a particularly steep decline.

That said, it wasn't unrelentingly bad news. The report also documented a few conservation success stories. The tiger population in Nepal has been rebounding after the Nepalese government cracked down on poaching. And mountain gorillas are rebounding sharply in Rwanda, Uganda, and the Democratic of Congo, thanks to a thriving new "gorilla tourism" industry.

4) Every single ocean now has a massive swirling plastic garbage patch

Plastic has become an unavoidable feature of modern life. But where does it go when we throw it out? Some ends up in landfills. Some gets recycled. But this past year, we learned that a surprising amount ends up in the ocean.

Most people have already heard of the Great Pacific garbage patch — a giant patch of plastic trash that's accumulated in a swirling subtropical gyre in the northern Pacific Ocean. But scientists recently discovered that there are at least five of these floating garbage patches around the world.

These patches aren't visible from space — or even necessarily from a passing boat. Over time, the plastic bits get broken down into ever smaller pieces as they get battered by waves and degraded by the sun, and many of the pieces are bobbing just below the surface. But they're there.

Another recent study in Science calculated that between 5 million and 13 million metric tons of our plastic waste makes it into the ocean each year. Surprisingly, we still don't know where it all goes — only about 1 percent ends up in those patches. One possibility is that marine creatures are eating the rest of the plastic and it's somehow entering the food chain, possibly with adverse effects on marine life. But it's a genuine mystery.

5) Antarctica is now melting faster than expected

Scientists have known for decades that humans are warming the planet by adding greenhouse gases to the atmosphere through the process of burning fossil fuels, cutting down trees, and expanding agriculture. The consequences, however, are still coming as a surprise.

Over the last year, for instance, a number of studies have indicated that the massive ice sheet that sits atop Antarctica is now melting faster than we'd previously realized. In March 2015, a major survey revealed that the ice shelves that keep those massive ice sheets hemmed in are now thinning at an alarming rate, eroded by warm water underneath.

What's more, a set of studies last year indicated that at least six of West Antarctica's glaciers appear to be melting irreversibly. That is, even if we stopped emitting carbon-dioxide tomorrow, some of that ice appears destined to slide into the sea over the coming centuries. That will push up global ocean levels, possibly by several feet or more. But exactly how much sea levels rise, and how quickly, largely depends on whether we continue to speed up global warming — or stop it.

6) Good news: The ozone layer is finally starting to heal

Not everything we learned last year was dire. We also got encouraging news about the ozone layer, a reminder that it's possible to stop environmental catastrophes before it's too late.

Back in the 1970s, scientists first realized we were rapidly depleting Earth's stratospheric ozone layer, which protects us from the sun's harmful ultraviolet rays. The culprit? Chlorofluorocarbons (CFCs) — chemicals that were widely used in refrigerators and air conditioners.

Between 1979 and 2013 these chemicals had chewed a massive "hole" in the ozone layer above Antarctica, and the damage was poised to spread further north. Without the ozone layer's protection, more and more people would be exposed to UV rays, and skin cancer rates in many places might have soared.

Happily, this apocalyptic scenario never came to pass. Scientists uncovered the problem in time. Under the 1987 Montreal Protocol, world leaders agreed to phase out CFCs, and eventually the hole in the ozone layer stopped expanding. Last year, a UN assessment found that the ozone layer was finally starting to heal — and should be back to its 1980 levels by 2050 or so.

7) We learned humans have been radically altering the Earth for longer than we thought

There's little question that humans are now the dominant force shaping Earth. But in recent years, scientists have been wondering when we became so dominant. Was it when we began exploding nuclear bombs? Was it the Industrial Revolution? Or was it earlier?

Increasingly, many researchers are pushing for an earlier date to mark the start of the "Anthropocene," the proposed term for the epoch when humans became the dominant force of change to the planet. Back in March 2015, in a paper for Nature, two scientist argued that the year 1610 was a good place to mark the start of when human influence was first felt globally.

Why 1610? That year, greenhouse gas concentrations in the atmosphere dipped drastically, by 7 to 10 parts per million. This was driven by sweeping changes that followed Columbus's landing in North America. New species were being introduced irreversibly into both continents. European diseases like smallpox killed tens of millions of people in North America. Agriculture collapsed, and forests were making a comeback, absorbing more carbon dioxide. Humans were having a truly global impact.

Not everyone agrees with that start date for the Anthropocene, however. In a commentary in Science in April 2015, a team of four researchers countered that we were radically altering the planet long before that. Humans were clearing forests for agriculture 7,000 years ago, leading to meaningful rises in carbon dioxide emissions. Likewise, the spread of rice farming 5,000 years ago appears to have led to significant rises in methane emissions.

There continues to be a lot of debate about when, exactly, we should mark the start of the Anthropocene. But it's a good reminder that while Earth Day — and environmentalism — are modern phenomena, the issues they describe go back many centuries, if not longer.