▲基本结构分析：

主句：they soon ran into serious trouble

从句：Though, at first, Byrd and his men were able to take a great many photographs of the mountains that lay below（让步状语从句）

▲让步状语从句分析：

在 Though, at first, Byrd and his men were able to take a great many photographs of the mountains that lay below 这一让步状语从句中，又包括有一个定语从句that lay below，该定语从句所修饰的先行词是 mountains，the mountains that lay below 的意思是“下面的群山”。

▲lie below 的意思：这里的 lie 是不及物动词(lay是它的过去式)，意思是“位于”；below 是副词，意思是“在下面”，所以 lie below 的意思是“位于下面”，the mountains that lay below 的意思是“位于下面的群山”。

▲句意：虽然开始时伯德和他的助手们拍下了飞机下面连绵群山的大量照片，但他们不久就遇到了大麻烦。