Factual error: Trevor - a Professor of Geology - boasts about having an article published in Scientific American, and that is not something any scientist would do. Scientific American is looked upon with slight disdain by the scientific community, considered to be a populist crowd pleaser. It is not even peer reviewed. Considering that he has just turned the geological and archaeological worlds on their heads he would have been better off publishing in Journal of Geological Research or Geology, both prestigious professional journals.
Factual error: Hannah's full name is Hannah Ásgeirsson. Icelandic surnames are (Father's name) sson for boys and (Mother's name) dottir for girls, so Ásgeirsson would be the surname of the male child of a man named Ásgeir. Her name should be Hannah Sigurbjörnsdóttir.
Factual error: While magnesium is the seventh most abundant element in the Earth's crust, it never occurs naturally in the uncombined state because it is too reactive. Therefore, there are never veins of magnesium metal to be ignited. Magnesium metal (uncombined state) will burn if ignited.
Factual error: When Trevor, Sean and Hannah enter the cave full of emeralds, rubies and diamonds, you can clearly see that these precious stones are not in their rough, natural forms. Instead the diamonds are crystal clear and look shiny and polished. In reality they would be rough and have a murky yellow tint to them. (00:32:40)
Factual error: It is impossible to receive a cell phone call at the center of the earth. There is no way that a signal can penetrate thousands of kilometers underground like that.
Factual error: Diamonds, emeralds, and rubies do not occur together, they form under very different conditions.
Factual error: The mountain guide seems to prefer SureFire flashlights, carrying an M6 Guardian. Later on, they find Max's camp in the centre of the Earth and the guide picks up another dirt-covered M6 that belonged to Max. Max went missing in 1997, 3-4 years before SureFire produced that light, so he couldn't have had one.
Factual error: When Sean is crossing the chasm by jumping on the 'magnetic rocks', he jumps onto the edge of several of the rocks. Assuming that the rocks could actually float in the magnetic field as shown, standing on the edge of one of them would cause it to immediately flip.
Factual error: The crater of Mount Vesuvius has been fully blocked with rocks and soil since its last explosion few decades ago. There are no openings in it and no geysers shooting out.
Answer: A mountain.