The Battle of Jutland was the decisive naval clash of the First World War, pitting the German High Seas Fleet against the Royal Navy's Grand Fleet in an all or nothing battle for supremacy and survival. At the end of the war, the defeated German fleet was scuttled at Scapa Flow. Or so we thought. New evidence suggests that wrecks in Portsmouth harbour, previously thought to be nondescript vessels, are in fact German veterans of the Battle of Jutland, scrapped at Portsmouth rather than Scotland. Dan Snow joins a team of marine archaeologists to explore these wrecks. But will he make it across the treacherous mud to reach them?