Final answer:
The word 'enigmatic' means mysterious or difficult to understand, as indicated by the context clues surrounding Stonehenge and Area 51.
Step-by-step explanation:
Based on the sentence provided, 'places such as Stonehenge and Area 51 are enigmatic', the word enigmatic can be inferred to mean mysterious or difficult to understand. The context clues from the sentence include specific examples like Stonehenge and Area 51, both of which are known for their mysteries and lack of full understanding by the public and researchers alike. Stonehenge, a prehistoric monument in England, has puzzled scientists for centuries regarding the purpose of its construction and alignment with celestial bodies. Similarly, Area 51, a highly classified remote detachment of Edwards Air Force Base within the Nevada Test and Training Range, is famous for its secrecy and associated conspiracy theories, which add to its enigmatic nature.