You must keep alert when you are in a completely unknown environment.

在一个完全陌生的环境里，你必须随时保持警惕。

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We meet today with unprecedented challenges, all of which are completely unknown.

我们今日遇到挑战前所未有，所有的情况完全陌生。

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Clearly there is a lot going on deep in the Earth that is completely unknown to science.

显然，地球深处发生了许多科学完全未知的事儿。

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She died in eighteen eighty-six, at the age of fifty-five, completely unknown to the world.

VOA: special.2010.07.18

Then on April thirtieth, two thousand seven,a huge fire of unknown cause almost completely destroyed South Hall.

VOA: special.2009.08.21

Cranium was a completely unknown board game company until it forged a massive distribution relationship with Starbucks.

FORBES: An Entrepreneurs Handbook To Slaying Goliath

With a project this big, the most interesting results may be unintentional--and completely unknown at this point.

FORBES: Totally Wired U.

At least one-third of the species that inhabit the world's oceans may remain completely unknown to science.

ENGADGET: Scientists estimate at least one third of marine species remain unknown to humans