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: When falling into the deep, deep hole, the three would be killed upon impact, and the fact that they follow water will not change anything. They will still be falling with more than 180 km/h. The water isn't slowing them down, as nothing is holding the water up. So it will be free-falling too - with more or less the same speed.
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.
Answer: A mountain.