Detective Murdoch investigates a series of robberies where the thieves have dug through the the ground and into various shops or banks. The thieves only steal diamonds however. Murdoch suspects the thief is using some type of boring device and that he and Julia may have actually felt it burrowing under the ground the night of the first robbery. Constable Crabtree develops his own theories after interviewing Elva Gordon, a deep cave explorer who believes the Earth is hollow and that there might be an entire world of people living underground. Crabtree now believes "mole people" may be at work. That theory doesn't impress Murdoch who decides to use modern technology to track the boring machine.
- George once dug out his Aunt Magnolia's cellar. It took me three Saturdays to get the length of the house. So a city block would take a few weeks
- George becomes an avid believer of the Hollow Earth theory.
- George complains of how he always has to dig for Murdoch, but Murdoch tells him that he is much better at digging than he is.
- The concept of a "Hollow Earth" has been prevalent throughout various mythologies and folklore in the ancient world. During the 19th century John Cleves Symmes Jr., a former soldier, gained notoriety for his theory that earth is actually a hollow shell. John Quincy Adams was supportive of a quest to go and find the Hollow Earth, but his successor, Andrew Jackson quickly put an end to any and all possibility of such an expedition.
- The process of electroplating is partially addressed as the procedure where metals are broken down to molecules, then bonded with another surface, covering the entire area.
- Seismometer was coined by David Milne-Home in 1841, to describe an instrument that measure motion of the ground, including those of seismic waves generated by earthquakes, volcanic eruptions, and other seismic sources, designed by Scottish physicist James David Forbes.
- George introduces Emily to the new Hot Sausage aka Hot Dog and orders the 'Coney Island' for them in the end scene.
- Showrunner Peter Mitchell explains how the storyline came about: “One of us [ in the Writers' Room ] read about the American president at the time who believed in mole people. He in fact wanted to fund an expedition to go the centre of the Earth to find mole people! ”
- There are no murders in this episode, unlike the rest of Season 7.
- The Graphizer voted #4 of 5 Favourite Murdoch Inventions  by The Arts Guide 8/20/15 Adnan M.
- As of 2017, Lauren Lee Smith portrays Frankie in the Frankie Drake Mysteries (2017-2021) who appears in the series' premiere online web series with Brackenreid and Watts which takes place in the future – Toronto 1921.
|Murdoch Mysteries Season 7|
|"Murdoch Ahoy" • "Tour de Murdoch" • "The Filmed Adventures of Detective William Murdoch" • "Return of Sherlock Holmes" • "Murdoch of the Living Dead" • "Murdochophobia" • "Loch Ness Murdoch" • "Republic of Murdoch" • "A Midnight Train to Kingston" • "Murdoch in Ragtime" • "Journey to the Centre of Toronto" • "Unfinished Business" • "The Murdoch Sting"•"Friday the 13th, 1901"•"The Spy Who Came Up to the Cold"•"Kung Fu Crabtree"•"Blast of Silence"•"The Death of Dr. Ogden"|
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