Literal:
* The fortress walls were impenetrable, making it impossible for the enemy to breach its defenses.
* The jungle's dense foliage created an impenetrable barrier, making it difficult to navigate.
Figurative:
* Her gaze was impenetrable, revealing nothing of her true thoughts.
* The mystery writer's plot was impenetrable, leaving the reader baffled until the very end.
* His logic was impenetrable, making it impossible to reason with him.
Formal:
* The scientific research was impenetrable to those without specialized training.
* The legal jargon used in the contract was impenetrable to the average person.
Remember, "impenetrable" usually implies something that is very difficult or impossible to understand, see through, or pass through.