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:

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