The ocean seems invulnerable because it is vast and under-explored.

海洋看似坚不可摧因为它广博而且未经充分开发。

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They explored the gradations between "cyclical" and "structural" unemployment, and the conditions under which one or the other arises or gives way to the other.

他们探究了“周期性”和“结构性”失业之间的渐变过程和两者产生、转换的条件。

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He voyaged sixty thousand miles under the sea, toured around the moon, explored the center of the earth, and chatted with natives in Australia.

他还到海底航行六万英里，他环游过月亮，他去过地心探险，他还去过澳大利亚，并与当地的土人聊天。

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He says the main reason why India remains under-explored is that mining leases and exploration licenses have not been given for large areas, even when there are indicative gold reserves.

BBC: Golden opportunity for old Indian mines