BBC

Sherlock's season four finale went all out to blow the socks off its audience, as it delivered an action-packed 90 minute climax that threatened the lives of just about all of our favourite characters at one point or another. It is an episode that has proved controversial with fans, even though pretty much all are praying it won't turn out to be the final ever episode of the show, as showrunners Steven Moffat and Mark Gatiss have threatened.

With Sherlock, John and Mycroft trapped in an underground facility at the whim of a psychotic inmate, it was more like a horror or action movie than Sherlock has ever been. And, whatever you thought about it, it was certainly very far removed from the traditional armchair detective stories written by Arthur Conan Doyle.

While it might have been of a very different spirit to Doyle's work, however, The Final Problem was still stuffed to the rafters with nods and references to the original Sherlock Holmes stories - along with a few callbacks to some other bastions of pop culture.

Read on for 20 easter eggs you might have missed in the season - series? - finale of Sherlock.