SUMMARY: Scientists are fairly certain that the Earth went through a snowball glaciation 600-800 million years ago, when the entire planet was locked in snow and ice. One new theory to explain this extreme cooling is the possibility that the Solar System passed through an interstellar cloud of dust and gas, which obscured the light from the Sun. Even if the cloud wasn't thick enough to obscure light from the Sun, it could have enabled charged particles to pass into the Earth's atmosphere and destroy the ozone layer. These clouds are huge, and it would take the Solar System 500,000 years to pass through one.

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