With climate change, plastic pollution and a potential sixth mass extinction, humanity has made some incredible messes in the world.

But when people, political factions and nations have pulled together, they have also cleaned up some of those human-caused environmental problems, including healing the ozone hole, clearing perpetually smoggy air and saving many species from the brink of extinction.

“We can be good at cleaning up our messes, it’s whether or not we choose to be and what we prioritize,” said Michigan State University environmental sustainability researcher Sheril Kirshenbaum.

For Earth Day, The Associated Press asked more than 25 environmental scientists and policy experts, including two former U.S. Environmental Protection Agency chiefs and the current director of the United Nations Environment Programme, to share their top stories about environmental problems that the world fixed.

“There are some amazing success stories,” said Stanford University environmental scientist Rob Jackson. “It’s easy for us to get tunnel vision with everything going wrong, and there is a lot that needs to change quickly. But it’s wonderful to remind ourselves that other people in the past have succeeded and that society has succeeded too, both nationally here in the U.S. and also internationally.”

Here are the four successes mentioned most often and a key aspect that so many ecological wins have in common.

Healing the ozone hole

Fixing ozone depletion was by far the top choice of scientists, officials and environmental policy experts.

“It was a moment where countries that usually compete with each other grasped the collective threat and decided to implement a solution,” former EPA chief Carol Browner said in an email.

Cleaner air and water

In the United States and much of the industrialized world, the air is much cleaner and clearer than it was 50 or 60 years ago when major cities like Los Angeles were choked with smog and even more dangerous microscopic particles in the air. And lakes and rivers were dumping grounds, especially around Ohio, Michigan and Canada.

“We would go to Lake Erie when I was young… and play on the beach and there would be dead fish everywhere. We would have dead fish fights,” Stanford’s Jackson said.

In the United States the Clean Air Act of 1970 and its follow up in 1990 with EPA regulations “effectively cleaned our air,” UNC’s West said. A similar law passed in the 1972 for water

Endangered species

The bald eagle, American alligator, peregrine falcon, Canada geese and humpback whales are each environmental success stories.

All were once on the brink of extinction, put on the endangered species list for protection. Now they are all of the protected list and in some cases they are so abundant that people consider them a nuisance or they cause problems for other species.