Unknowable Episode 20: The Zodiac Killer

This week we talk about the yet-to-be-solved case of The Zodiac Killer. He murdered a confirmed 5 people in northern California from 1968-1969 and sent numerous letters to local police stations and newspapers, including many mysterious ciphers (only one of which has been cracked to this day). Was it one man who committed all these crimes, or several unconnected murders that only became linked when a separate entity started sending the letters? Did he kill 5 people or the 37 he claimed? WHO is the Zodiac?