Related Clues:

• Hard to fathom
• Mysterious
• Hard to understand
• Hard to figure out
• Cryptic
• Puzzling
• Baffling
• Very strange
• Tough to crack
• Hard to read
• Tough to figure out
• Cryptic, mysterious
• Mysterious like Mona Lisa's smile
• Mysterious or puzzling
• Mysterious or puzzling, like Mona Lisa's smile