unfathomable

1. ADJ If you describe something as unfathomable, you mean that it cannot be understood or explained, usually because it is very strange or complicated. (事物)不可理解的

For some unfathomable reason, there are no stairs where there should be. 因为某种不可理解的原因，该有楼梯的地方却没有。

2. ADJ If you use unfathomable to describe a person or the expression on their face, you mean that you cannot tell what they are thinking or what they intend to do. (人、表情)莫测的[文学性]

...a strange, unfathomable and unpredictable individual. ...一个奇怪的、高深莫测且难以预料的人。