If you describe something as unexplained, you mean that the reason for it or cause of it is unclear or is not known.
例句
The demonstrations were provoked by the unexplained death of an opposition leader...一位反對派領袖莫名身亡，引發了示威游行。Soon after leaving Margate, for some unexplained reason, the train was brought to a standstill...離開馬蓋特不久，不知什么原因，火車突然停了下來。
英漢詞典釋義
adj.
不清楚的,使人莫名其妙的
There are many tales of the unexplained.有許多無法解釋的傳說。