1. 潜在的；隐藏的；深层的；根本的
    The underlying features of an object, event, or situation are not obvious, and it may be difficult to discover or reveal them.

    e.g. To stop a problem you have to understand its underlying causes...
           要想杜绝问题的发生，必须弄清它的深层原因。
    e.g. I think that the underlying problem is education, unemployment and bad housing.
           我认为深层的问题在于教育、失业和糟糕的住房条件。

2. 表面下的；下层的
    You describe something as underlying when it is below the surface of something else.

    e.g. ...hills with the hard underlying rock poking through the turf...
           坚硬的下层岩石穿过草皮凸出来的山丘
    e.g. Cars were covered with clear-coat finish at the factory to protect the underlying paint from fading.
           工厂给汽车上了透明罩漆，以防底漆褪色。