习语口语对话大全(MP3+中英字幕) 第247期:once in a blue moon

once in a blue moon

极为罕见

A:The river is much higher than the ground.

A:河比地高得多。

B:How dangerous!

B:多危险呀!

A:Yes, it is.

A:是呀。

B:What if the river overflowed its bank?

B:河水要是涨过了岸，怎么办?

A:That would be disastrous.

A:那可遭殃了。

B:The whole city will get flooded?

B：整个城市就会被水淹了?

A:Of course. But that happens only once in a blue moon.

A:就是，不过那百年也不见得会遇上一次。