这个句子只有一个从句.
（主句,先行词是sound）which (they found插入语) was caused by a small waterspout shoot down into a pool from the roof of the cave.
（限制定语从句,引导词是which,在从句中做被动主语）
英语中,I think(feel/find/believe/imagine/suppose/consider)常可用作插入语.
翻译：在洞穴的这个部分,他们可以听见一种连续不断的轰鸣声.他们发现这是山洞顶部一个小孔里喷出来的水柱跌落到水潭中发出的声音.