冰层推进
Ice advance
以上为机器翻译结果，长、整句建议使用 人工翻译 。
为什么冰层会周期性地推进，为什么又会消退，这还是个谜，冰河学家们正试图揭开谜底。
Why the ice periodically advances - and why it retreats again - is a mystery that glaciologists have only just started to unravel. Here's our recap of all the back and forth they're trying to explain.
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