In this episode, Troy, Phones, Marina and Atlanta are all supposed to have leave at the same time, which seems unusual. (They take a dig at the woman's packing sterotype. Marina is shown bouncing on a suitcase to get it to close. Atlanta comments that she hopes what they have is sufficient, as Troy said they would be traveling light. She then strolls over to a huge pile of luggage.)
Leave is canceled, because some scientists who have spent untold years digging down through the crust of the Earth to the mantle, have discovered another ocean down there. Stingray is required for exploration.
This is where it gets ridiculous. Earlier, they had an episode where they had to follow an attacking alien very deep into the ocean, and very nearly destroyed Stingray because of the pressure. Well, going down below the Earth's crust would surely subject them to even more intense pressure, but they showed no sign of it.
They find that the ocean has very radical tides, with the water disappearing completely (and going just where, may I ask?). They get out to explore, all of them wearing oxygen, including Marina, since "there's no air out there." Well, that lizard had to be breathing something, and plants require some oxygen (and release still more of it.) The tide comes back, they make it back to Stingray, and then can't find their way back to the exit and start running out of air. Just how were they expected to explore an unknown ocean with such a limited air supply? Then, they discover that the scientists wasted all those years, effort and money digging through, because there just happens to be a NATURAL shaft coming clear through the crust, a shaft that's right under a nice, inhabited tropical paradise. No indication that they reported their findings either to the scientists or to Headquarters, they just hauled out on the beach and started relaxing.
By the way, Troy had better expect a long frozen period from Atlanta. Getting a photograph postcard from them, showing Troy and Marina enjoying themselves with a casual "Wish you were here" note but no suggestion that she fly out to join them...Troy is either incredibly thoughtless or incredibly mean.
When it starts getting warm down there, Troy says that they must be close to the center of the Earth. They had only gone down two miles before going through the crust, and they couldn't have gone must further down in the sub. Methinks that the center of the Earth was maybe just a little further down than Troy thought.
They knew little about the Earth back then I'm pretty sure they didn't know about plate tectonics so they did a pretty good job here. As for Troy saying they were close to the centre of the Earth he was probably exaggerating or maybe he thinks of the mantle as the centre.
The science people waste who know how much time and money burrowing down to this sea when there's a natural shaft somewhere already, whoops.
There would be air down there because of the natural hole but they had no way of knowing that. For all they knew the plants lived off the oxygen in the water and lasted out the dry spell. Also they didn't see the lizard. Whilst there quite clearly is air down there they had no way of knowing that.
It was pretty rough on Atlanta, also didn't commander shore want Stingray back instead of leaving it in a lake on an island in the middle of nowhere. Considering that they'd have to lift it out of there Atlanta could have gone out with the recovery crew and stay there.