Plot hole: We don't see any herbivores in the underground world - just fish and little birds. A T-Rex would need lots of big game to survive.
Plot hole: While the slopes on the mine tracks make for a thrilling downhill ride, the cars loaded with ore could never have made it back up the tracks.
Plot hole: Sometimes the cave heats up to the point that the ocean boils. One of these events killed Max a few years before, and the second half of the movie revolves around the main characters trying to reach a geyser, which is said to be the only way out, while another event is beginning. No explanation is provided for how the flora and fauna of the cave survive such events.
Answer: A mountain.