Synonyms: mysterious, esoteric, arcane, occult, inscrutable

These adjectives mean beyond human power to explain or understand. 

• Something mysterious arouses wonder and inquisitiveness:
  ... "The sea lies all about us.... In its mysterious past it encompasses all the dim origins of life" Rachel Carson. 
• What is esoteric is mysterious because only a select group knows and understands it:
  ... a compilation of esoteric philosophical essays. 
• Arcane applies to what is hidden from general knowledge:
  ... arcane economic theories. 
• Occult suggests knowledge reputedly gained only by secret, magical, or supernatural means:
  ... an occult rite. 
• Something that is inscrutable cannot be fathomed by means of investigation or scrutiny:
  ... "It is not for me to attempt to fathom the inscrutable workings of Providence" Earl of Birkenhead.

.... Am-Heritage