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Earth Checkup: 10 Health Status Signs

With Earth Day on the way, LiveScience is taking a look at the state of Earth's systems and inhabitants -- a planetary checkup to see what things are going right and which areas could use some improvement. From both poles to the depths of the sea, from the air we breath to the air that keeps us from burning up, we're understanding more and more our impact on the planet and ourselves.

-- Andrea Thompson, Senior Writer