10 Most Shocking Unsolved Crimes

Cold cases are occasionally cracked after years of frosting over, but even more go unsolved, and the longer they remain open, the more notorious they become.

Murders, disappearances and other crimes the authorities can't crack often strike a chord with conspiracy theorists, armchair investigators and amateur internet sleuths who think they're smarter than the law, and the debate they spark among these circles usually burns hot.

Modern history is awash with unsolved crimes, sometimes because the perpetrators planned it meticulously and covered their tracks, at other times because the investigators made critical errors, and occasionally it's a little of both.

Such cases, however, stretch back across the ages to a time when records had only just begun. Every once in a while, modern advancements in fields such as forensic science yield breakthroughs in these enduring criminal mysteries, but cases that remain unresolved for more than a decade usually stay that way.

Even after that length of time and longer, the internet never seems to give up trying to put these mysteries to bed...

The killing spree began on 20 December, 1968 when a couple was shot dead while sitting in a car on Lovers Lane in San Francisco, and it lasted for 10 months.

The Zodiac is known to have killed at least five people, but the fact he got away with these crimes is only partly why they're so notorious. This shadowy serial killer toyed with police and reporters at the time, goading them with cryptic, coded messages.

Doubt over the authenticity of the notes was banished when their author mailed in a piece of bloodied clothing that had belonged to one of his victims.

Several suspects have been considered over the years, but the Zodiac's identity remains unknown and the case is still officially open in Napa County and Riverside.

David Fincher's 2007 film Zodiac, starring Jake Gyllenhaal and Robert Downey Jr, explores the case and the impact it had on those who were directly involved with it.