The Crystal Cylinder of Stonehenge was one of the mysteries of Stonehenge.
The cylinder was found at Stonehenge in 1936 after a freak electrical storm toppled one of the stone blocks, splitting it open to reveal the crystal cylinder inside. Collected by the British Government, officials believed that the message on that cylinder was of vital importance to the free world but Nazi agents led by Emil Loeb were also interested in the artifact and attempted to steal the cylinder from Indiana Jones and Karen Mays, who had been brought in on the British "Gateway Project" to decipher the symbols.
Jones and Mays eventually found that the symbols of the cylinder were carved by creatures that pre-dated man who had used Stongehenge as a portal to flee the planet to another realm, but promised to one day return. However, due to the machinations of Hauptmann Loeb, Jones found he had to destroy the cylinder against his better judgement and the gateway was sealed.