Earth's 8 Biggest Mysteries

When the first Earth Day was held in 1970, geologists were still putting the finishing touches on plate tectonics, the model that explains how the Earth's surface takes shape. More than 40 years later, many riddles still remain when it comes to our planet. Here are some of Earth's biggest unsolved mysteries.

8. Why are we all wet?

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Happy Birthday, Hubble! 10 Epic Photos from the Iconic Space Telescope

The Hubble Space Telescope is celebrating its 25th anniversary. The iconic observatory, which was launched into space on April 24, 1990, has had its share of adventures over the years — including a faulty mirror (later repaired) and visits from astronauts on servicing missions. For the past 25 years, Hubble has given us new perspectives on planets across the solar system and jaw-dropping views of locations across the universe. Here are some of the space telescope's greatest hits.

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