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In page Greenhouse and icehouse Earth:
"Some biologists claim that a complete snowball Earth could not have happened since photosynthetic life would not have survived under many meters of ice without sunlight. However, sunlight has been observed to penetrate meters of ice in Antarctica[citation needed]. Most scientists[citation needed] now believe that a "hard" Snowball Earth, one completely covered by ice, is probably impossible. However, a "slushball Earth", with points of opening near the equator, is considered to be possible.
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