In days of yore, a mighty runmbling was heard in a mountain. It was said to be in labor, and multitudes flocked together, from far and near, to see what it would produce. After long expectation and many wise conjectures from the by-standers -- out popped a Mouse!

the story applies those magnificent promises end in a paltry performance.

古时候，在一座山里发出了一阵隆隆巨响，据说这是大山要临产了。人群从四面八方聚集起来，观看大山会生出什么后代。大家企盼了许久，并且作了许多聪明的推测，结果从那里跳出一只老鼠。
这个故事说的是，雷声大，雨点小。

词汇：

In days of yore 古时候

in labor 临产

multitudes flocked together 人群聚集

by-standers 旁观者

out popped 跳出

paltry 微不足道的