Gazing at the Ocean and Sighing

One autumn, the rivers flooded, leaving a vast expanse of water everywhere. Seeing this, the god of the rivers was filled with pride at his vast domain. He then journeyed to the Northern Sea. When he saw the mighty ocean stretching to the horizon, he realized how puny he actually was, and sighed with disappointment. 

有一年秋天，河里发了大水，河水淹没了田地和树林，到处都是白茫茫的一片。河神看到这种景象，自以为很伟大，很了不起。他顺着河水来到了北海，望见海洋无边无际，水几乎和天空连在一起，这时他才感叹起来，觉得自己真是太渺小了。 

This idiom originally meant feeling one's own insignificance upon seeing another's might. Now it is mostly used to indicate being able to do nothing but sigh in the face of a huge task. 

“望洋兴叹”原来的意思是：看到人家的伟大，才感到自己很渺小。现在多用来比喻做事力量不够，无从着手，而感到无可奈何。