Continuity mistake: When walking in the subterranean world, Trevor holds and reads from the Jules Verne Book. It is completely dry, even though they have just been swimming. A wet book stays wet for a long time, and the pages stick tightly together. And yes - they were long enough in the water for the book to get wet - and no, it was not in a watertight container.
Character mistake: When in the mineshaft Trevor says that magnesium is used in gunpowder. That is not correct.
Continuity mistake: As Trevor and Hannah and the boy are traveling up the volcano on the cushion of steam, Trevor's arm constantly changes position instantly with different shots. Sometimes his arm is across Hannah's chest, then it's under her head, and back across her chest.
Continuity mistake: When the Italian farmer starts to shout, Trevor raises his right arm to comfort Hannah. In the next shot it's down, and he doesn't raise it until much later.
Continuity mistake: The sweat stains on the front of their shirts keep changing throughout the movie.
Other mistake: Max was killed in the beginning falling down a crack in the earth into hot lava, but then later they find his bones and bury him.
Continuity mistake: After Trevor and the others are submerged in the water they have soaked everything, but the book is intact and dry moments after that.
Character mistake: Trevor tells Sean to "eat his trilobite to keep his strength up." However, Trevor, a geologist, pronounces trilobite incorrectly as "TRIL-o-byt" instead of the correct pronunciation "TRY-luh-byt." The name derives from the fact that the creatures have 3 (tri) lobes. Tri - for 3 not Tril - for 3.
Continuity mistake: Trevor is saved from falling down a cliff and sees a sign covered in dust. He gives it to Hannah and in the close-up most of the dust is gone.
Continuity mistake: The Italian farmer runs fast to see the crash. When the angle changes he's moved several meters behind where he was standing, and is now walking.
Continuity mistake: While crashing against the vineyards, the amount of grapes stain on all of the 3 guys' faces is not continuous between shots. It appears/disappears randomly.
Continuity mistake: After exiting the volcano, the group starts to fall down and they all shout in desperation. Then it cuts to a close-up where the cry is faintly heard, but none of them are shouting.
Continuity mistake: When the kid gives the diamond to the Italian man, the way the farmer holds it differs between shots.
Continuity mistake: After Trevor is saved from falling down a cliff, the way Hannah holds the No entry sign differs between shots.
Continuity mistake: Trevor arrives at the lab and Leonard has made a popsicle. The amount of ice on it increases or decreases randomly between shots.
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.
Continuity mistake: After having flown up the volcano, and they start falling, while still lying in the T-Rex jaw, they should have been weightless, but they still lie down.
Continuity mistake: The black hose next to Leonard swaps from stiff to bent down between shots.
Continuity mistake: When Trevor brushes his teeth, the towel on the left changes the way it's wrinkled between shots.
Continuity mistake: In half a second, Leonard's hair swaps from messy to brushed to the right when Trevor arrives at the lab, despite him having both hands busy.
Answer: A mountain.