1. Suddenly they came to a waterfall which droped into an underground lake at the bottom of the cave.
    他们突然遇见一条瀑布，它直泻入洞底的地下湖里。

2. Suddenly they came to a waterfall which dropped into an underground lake at the bottom of the cave.
    突然，他们来到一道瀑布前，那瀑布奔泻而下，注入洞底一处地下湖里。