Despite the importance of sleep, its purpose is a mystery.

Psycho-horror films dealing with the mysteries of human psychology are popular these days.

Where the treasure is hidden is still a mystery.

Her disappearance gave zest to the mystery.

It is still a mystery why she killed herself.

What happened to them is still a mystery.

At last, he found a clue to the mystery.

He had no difficulty in explaining the mystery.

He turned up a key to the solution of the mystery.

He looked into historical documents to solve the mystery.

He found a clue to solve the mysterious affair.

The cause of his death still remains a mystery.

His scientific discovery unlocked many mysteries.

There's something mysterious about him.

They are investigating the mystery of the universe thoroughly.

It is a mystery why he quit his job.

It is a mystery why he did not finish the poem.

How he escaped still puzzles us.

Whether he wrote it or not will always remain a secret.

Don't leave the riddle.