The device exploded underneath a van.
那个装置在一辆面包车底下爆炸了。
The part of something which is underneath is the part which normally touches the ground or faces toward the ground.
在下面; 在底部
She pulled the drawer out and examined the underneath carefully.
她拉开抽屉仔细查看底部。
They want to see what's underneath.
他们想看看下面是什么。