idiom · puzzling and difficult to interpret, especially regarding a person's behavior, expression, or whereabouts

那名间谍的行踪神秘莫测，连他的上司都联系不到他。

idiom · extremely mysterious and impossible to measure or predict, often used for natural phenomena or complex situations

大自然的力量神秘莫测，人类至今仍无法完全掌握其规律。