Karl Heinrich is a fictional character in the story "The Temple" by H. P. Lovecraft. He was a lieutenant-commander in the Imperial German Navy and commanded the submarine U-29 during the First World War. He was ruthless and arrogant, and often resorted to capital punishment against his own men. He regarded himself as culturally and intellectually superior even to his own crewmates, many of whom he regarded as irrational, superstitious, and weak-willed.
As related in the story, the crew fell victim to an apparently divine curse after sinking the British freighter Victory in the North Atlantic Ocean. As a result of this curse the submarine found itself in the grip of an inexorable current that carried the vessel southward and ever deeper. Along the way, the crew experienced supernatural phenomena as well as technical malfunctions, casualties, and an attempted mutiny.
Heinrich was the only crewman left alive when the U-29 eventually came to rest among the splendid ruins of an underwater city, presumed by him to be a remnant of the lost continent of Atlantis. Putting on his diving suit he explored the ruins and discovered a magnificent temple that glowed from within. Compelled by an unknown force, Heinrich entered the eldritch glow of the temple, never to be seen again. His written account that he had sealed in a bottle before entering the temple was eventually found on the coast of Yucatán, Mexico.