Scientists use "hole" as a metaphor for the area in which ozone concentrations drop below the historical threshold of 220 Dobson Units.

科学家们用“空洞”一词表示臭氧浓度低于220个多布森单位（多布森单位是用来度量大气中臭氧柱尺度的单位。

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The hole isn't literal; no part of the stratosphere is empty of ozone. But scientists use the hole metaphor to describe the area where ozone drops below a historical concentration of 220 Dobson Units.

这里所谓的“空洞”不是字面上含义，并不存在任何一处平流层完全没有臭氧，科学家们用空洞这一说法来描述这些臭氧浓度低于历史读数220多布森单位的区域。

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Two different worlds: Border guards stand in single file above, while below a West Berlin child smiles and points to a hole in the wall.

两个世界：上面边防兵站成一排，下面一个西柏林的孩子微笑着指着墙上的一个窟窿。

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His ship had a hole below the waterline the width of a football field.

CNN: Covering a cruise ship disaster

Apparently the string of logging tools, each one measuring something different, needs some room to take their readings so the "rat hole" has to extend below the measured area.

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Water cascades down the side of the mountain, providing hikers with a refreshingly icy plunge into the swimming hole below.

BBC: The Philippines�� secret sanctuary