Theory:Doctor Who television discontinuity and plot holes/The Hungry Earth


 * The central concept of this episode — that the miners have achieved a record by drilling to 21 km, thus threatening the Silurian habitat — is inconsistent with both Inferno and Journey's End. The Inferno Project achieved a depth of at least 20 miles, and the Osterhagen Project placed their nuclear devices beneath the crust of the Earth, which varies between 20 and 70 km in thickness.  Humans should therefore have aggravated the Silurians long before this episode.  The Osterhagen Project should have been particularly incendiary to Human/Silurian relations, according to the underlying logic of this episode.
 * The Silurians don't live underneath every square inch of the surface. Presumable the Inferno and Osterhagen projects just happened to not be anywhere near any Silurian encampments (and likewise for the Daleks' later attempt to drill all the way down to the Earth's core to remove it so they could take the planet for a joyride), while this one was directly over a major city.
 * Of course that still doesn't explain how 21km could be a record. The Osterhagen project may have deliberately chosen the thinnest parts of the crust (which is actually well under 5km in some spots, especially on the ocean floor--the real-life Project Mohole, the inspiration for Inferno, planned to drill off-shore for exactly this reason). But Inferno was intending to drill all the way to the core, and got as far as 20mi (32km).
 * Maybe Inferno was a well-kept secret. After all, most scientific projects aren't guarded by UNIT.