Current estimates suggest that half of all the living matter on Earth lives underground, at depths of up to 5km.
现在估计地球上大约一半的生物生活在地下深度约5千米以下的地方。
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The fear is that, at current consumption rates, that underground bunker will be empty within 20 years, leaving the earth almost helium-free by the end of the 21st century.