Why is The Hound of the Baskervilles a mystery?

Mystery literature involves a puzzling crime of some sort. This crime is often a murder or theft. What elevates such crimes to the realm of mystery is that they are seemingly impossible, or at least not readily understood by those not directly involved. The Hound of the Baskervilles is a great example of how a murder can be a mystery, as the only apparent answer is an impossible one.

In Sir Arthur Conan Doyle's The Hound of the Baskervilles, the men in the Baskerville family are rumored to be haunted by an ancestral curse. Most recently, Sir Charles Baskerville has been found dead and surrounded by the footprints of a giant dog. Dr. Mortimer fears his late friend's nephew and heir, Sir Henry Baskerville, might also be killed by the spectral hound haunting the family. Dr. Mortimer (and Sir Henry) seek the help of Sherlock Holmes to try and prevent this from happening. 

The circumstances of Sir Charles' death are apparently plain — he was frightened to death by the ghostly dog while on a walk. Is it really as simple as that, though? Why would the hound resurface after so many years to target the Baskerville family? The whole setting of the Baskerville estate adds to our sense of mystery and trepidation as we learn Dr. Watson hears sounds in the night, the butler behaves strangely, and a murderer is loose on the moors! 

As with all of Holmes's cases, and with all mysteries, the facts are revealed in the end. Although most people were prepared to accept a supernatural answer in the wake of no other explanation, Holmes reveals that Sir Henry's cousin has been using his massive dog, coated with glowing phosphorous, to quite literally scare the life out of his relatives! This man hoped that if he killed off everyone ahead of him to inherit the estate, he would be named heir and make himself at home on the Baskerville estate. Indeed, Holmes figured out this motive long before he really understood how the murders were being committed. 

Mystery stories often follow a pattern where, after the initial exposition, some evidence is introduced that both answers questions and poses new ones. In almost every Sherlock Holmes story, however, the evidence is not made apparent until the very end.