How well do you know your planet?

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"Those who contemplate the beauty of the earth

find reserves of strength that will endure as long as life lasts."

— Rachel Carson

Question 1 of 15

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About how many species have been identified on Earth so far?

Roughly 1.5 million species are known, but scientists estimate the planet has at least 8.7 million. That means more than 80 percent of Earth's species are still undiscovered. (Photo: Santiago Ron/Flickr)

Question 2 of 15

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What is the largest-known animal to ever live on Earth?

Argentinosaurus, diplodocus and megalodon were all huge, but none of those extinct animals measures up to the blue whale, which still exists and can weigh 330,000 pounds. (Image: Shutterstock)

Question 3 of 15

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This is the smallest animal from what taxonomic group?

The mouse lemur, just 10 inches from head to tail, is the smallest primate on Earth. It's also a mammal, but not the smallest. That title belongs to the Etruscan shrew. (Photo: A.J. Haverkamp/Flickr)

Question 4 of 15

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What is Earth's largest source of breathable oxygen?

While plants produce lots of oxygen, the most prolific source is photosynthetic plankton, which generate about half of the breathable oxygen in Earth's atmosphere. (Photo: NASA Earth Observatory)

Question 5 of 15

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On average, how deep are Earth's oceans?

The average depth of Earth's oceans is about 4,267 meters, which is nearly 14,000 feet or 2.65 miles. (Photo: NASA Earth Observatory)

Question 6 of 15

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What is the tallest mountain on Earth, measured from base to peak?

Everest reaches the highest above sea level (29,000 feet), but most of Mauna Kea is underwater. From its base on the sea floor, the Hawaiian volcano is about 56,000 feet tall. (Photo: Vadim Kurland/Flickr)

Question 7 of 15

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About how many bones do sharks have?

Sharks don't have bones. They're cartilaginous fish, meaning their skeletons are made of cartilage. Since cartilage is less dense than bone, this saves energy. (Photo: Olivier Morin/Getty Images)

Question 8 of 15

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What animal performs the longest known annual migration?

The tiny Arctic tern migrates farther than any other animal, flying an average of 44,000 miles per year between the Arctic and Antarctica. (Photo: Didier Baertschiger/Flickr)

Question 9 of 15

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About how many plant species are edible for humans?

Of 350,000 known plants, people can eat about 80,000. But we usually don't: Just 30 plant species supply 95 percent of calories and proteins eaten by humans. (Photo: Justin Sullivan/Getty Images)

Question 10 of 15

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What percentage of Earth's plants and animals live in rain forests?

Rain forests cover less than 2 percent of Earth's total surface area, but they're home to about half of its plants and animals. (Photo: Shutterstock)

Question 11 of 15

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What is the deepest lake on Earth?

Lake Baikal in Siberia is Earth's deepest and oldest lake. It's 25 million years old, reaches a mile deep and also holds 20 percent of the planet's unfrozen freshwater. (Photo: Sergey Vladimirov/Flickr)

Question 12 of 15

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What is the longest river on Earth?

Winding 4,135 miles through Africa, the Nile is longer than South America's Amazon by only about 155 feet, although the Amazon is larger in terms of width and water volume. (Photo: Pixabay)

Question 13 of 15

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How old are the oldest fossils ever found?

The oldest fossil evidence of life comes from stromatolites, biochemical structures built by microbes. Some are 3.5 billion years old, just a billion years younger than Earth. (Photo: Julian Frost/Flickr)

Question 14 of 15

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Besides plants and animals, how many kingdoms of life exist on Earth?

Life on Earth is divided into six kingdoms: plants, animals, protists, fungi, archaebacteria and eubacteria. Pictured above is a rust fungus. (Photo: U.S. Department of Agriculture)

Question 15 of 15

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What is the longest-surviving species of human?

Homo erectus existed from 1.89 million years ago until 143,000 years ago. That's more than nine times longer than our species, Homo sapiens, has existed so far. (Photo: Wikimedia Commons)

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