In The Return of the King, Aragorn's company comes across the corpse of a man:
Before him were the bones of a mighty man. He had been clad in mail, and still his harness lay there whole; for the cavern’s air was as dry as dust, and his hauberk was gilded. His belt was of gold and garnets, and rich with gold was the helm upon his bony head face downward on the floor. He had fallen near the far wall of the cave, as now could be seen, and before him stood a stony door closed fast: his finger-bones were still clawing at the cracks. A notched and broken sword lay by him, as if he had hewn at the rock in his last despair.
What was behind the door, and why was the man so desperate to get inside? Did Tolkien ever share that information? This question has been bugging me ever since I originally read the books.