当这些生物死后，它们的外壳便会沉入水底并逐渐堆积成厚厚的石灰岩层，这就是为什么在石灰岩中经常可发现一些生物化石的原因。

When these creatures died, their shells fell to the bottom and gradually built up thick layers of limestone. For this reason, fossils are commonly found in much limestone.

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例如，过多的二氧化碳所造成的海洋酸度升高，进而影响了某些生物的结构。这些生物的外壳以后将变成地球的石灰岩。

For instance, acidity in oceans caused by extra CO2 affects the make-up of creatures whose shells will form Earth's future limestones.

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