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                           general:: 
                            mysterious 
difficult to explain or understand, and often strange or slightly frightening: 
• the mysterious disappearance of ships in the waters of the Bermuda Triangle • What’s that mysterious noise? 
puzzling 
difficult to explain or understand: 
• At first this must seem a puzzling question. • I found her attitude deeply puzzling. 
baffling 
impossible to understand, so that you feel very confused: 
• It was a baffling case. • Like many people, I find theroretical physics baffling. 
inexplicable 
formal impossible to explain: 
• For some inexplicable reason her mind went completely blank. • Some things in life are inexplicable. 
enigmatic 
formal mysterious and difficult to understand, but also often attractive or interesting – used especially about people, their expressions, or things they say: 
• She gave him an enigmatic smile. • an enigmatic remark 
cryptic 
formal having a meaning that is difficult to understand, and is not expressed in a clear direct way – used especially about things that people say or write: 
• I got a rather cryptic message from him. • cryptic comments 
be a mystery 
if something is a mystery, you cannot understand how or why it happens: 
• Four years later, his death remains a complete mystery. • It’s all a mystery to me. 
be shrouded/veiled in mystery 
if an event or a situation, especially one that happened a long time ago, is shrouded in mystery, it is mysterious because no one knows exactly what happened: 
• Stone age civilization, veiled in mystery as it is, has provided the greatest challenge to historians.
                        
                        
 
                        
                            Longman-Thesaurus