As the last ice age took hold of the Earth 100,000 years ago, the level of the sea fell 600 feet, in lock step with the permanent accumulation of snow and ice on the northern continents.
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Sixty-five million years ago an asteroid about ten kilometres (six miles) in diameter and moving at a velocity of 20 kilometers per second slammed into the Earth at a point near the modern city of Merida on the Yucatan Peninsula.
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