Since the 1930s, Lake Mead — a picturesque reservoir outside of Las Vegas — has been the site of over 300 drownings, making it one of the deadliest lakes in America. However, it wasn’t until recently that Lake Mead’s truly sinister history came to light. As environmental crises like drought, warming temperatures, and diminishing water supply have caused the reservoir to recede, people have discovered bodies in the lake. In a special edition of “20/20,” ABC News contributor Chris Connelly investigates the secrets of Lake Mead and details how authorities are using DNA evidence to crack cases that had long gone cold.