There are six people in the expedition attempting to escape. However, in long shots, there are seven people attempting to escape in stock footage from The Lost World (1960).
The paleontologist identifies Edaphosaurus as a dinosaur from the Jurassic Period of the Mesozoic Era. Edaphosaurus was from the Permian Period, the last phase of the Paleozoic. Although popular culture makes it an honorary "dinosaur," this animal was not a member of any true dinosaur order, so no paleontologist would refer to it as such. Furthermore, the creature shown in the adventure bears very little resemblance to an Edaphosaurus, which is drawn accurately in the reference book shown on screen.
The mile-deep underground world is warm and brightly lit. The warmth is, quite reasonably, attributed to volcanic activity but nobody questions or explains where the light comes from.
The underwater or underground world is at a depth of 4000 feet, that is nearly a mile down. The pressure there is around 120 atmospheres, enough to instantly crush any human to death, yet they exit their diving bell and walk around as if at the surface.
There is a visible wire lifting the lizard out of the lake.
There is a visible hook pulling the Seaview across the surface of the ocean.
One of the natives putting them in the cage was wearing new clean sneakers.
The natives living 4000 feet underground perform a ceremony at "high noon on the longest day of the year". They could not possibly know about time and lengths of days with no sun visible.
Admiral Nelson wears three stars on his collar showing him to be a vice admiral, but his sleeve strips of one large and three regular is that of a four star full admiral.
Nelson wants the age of the Edaphosaurus' skin dated, presumably using something like a Carbon-14 test. He asks Dr. Ziegler to use his "atomic clock" to do this, which then Ziegler tells his assistant to use. It's almost laughable to hear something misuse this term, but in the 1960's hardly anyone would have known what an atomic clock actually was.