The longest mountain range on Earth is underwater.
Why it is true
The Mid-Atlantic Ridge and the Pacific Ridge are part of a continuous system stretching over sixty-five thousand kilometres, far surpassing the Andes or the Himalayas.
Source: USGS
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This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.