1. 在…下面；在…底下
    If one thing is underneath another, it is directly under it, and may be covered or hidden by it.

    e.g. The device exploded underneath a van.
           该装置在一辆货车底下发生了爆炸。
    e.g. ...using dogs to locate people trapped underneath collapsed buildings.
           利用狗来寻找被困在倒塌的楼房下面的人

2. 在下表面；在底面
    The part of something which is underneath is the part which normally touches the ground or faces towards the ground.

    e.g. Check the actual construction of the chair by looking underneath...
           看看椅子的下面，检查一下它的实际构造。
    e.g. The sand martin is a brown bird with white underneath...
           崖沙燕是一种褐羽白腹的鸟。

3. 在内心深处；心底里
    You use underneath when talking about feelings and emotions that people do not show in their behaviour.

underneath的反义词

    e.g. He was as violent as Nick underneath...
           在内心深处，他和尼克一样暴躁不安。
    e.g. Underneath, Sofia was deeply committed to her husband.
           索菲娅从内心里对丈夫忠贞不贰。