Okay, so here are my thoughts on the whole Atlantis thing. It was written quite quickly and with a baby on my lap, so sorry if it's a bit messy.
I want(ed) to go Greek.
So, Atlantis has its name from Ancient Greek, where it means "Isle of Atlas". Atlas was the eldest son of Poseidon and Cleito and was made the king of Atlantis - he is also the titan who was sentenced to hold the sky after siding with the Titans in their war against the Olympians (Greek Gods), but this is not important to the story.
After the Atlanteans lost a war against the Athenians, Atlantis lost its favor with the Gods and the island was pulled into the sea by earthquakes and floods in a single day.
Supposedly, the Atlanteans were superior to the humankind somehow and I do plan to play with that and the myth of Undines together, since they were said to be humanoid, but lacking a soul and the only way for them to gain one, was to marry a human. The offspring of such a union would result in a human with a soul, but also with aquatic features of their Undine parent. Since Atlantis was completely surrounded by water and was the home of Poseidon, God of the Sea, Rivers, Floods, Droughts and Earthquakes, the Undine favored this island as their source of a spouse, making a large portion of the people there the offspring of an Undine/human union and even more of the people second generation of such, still with aquatic features. As such, a very large portion of the people of Atlantis would have been "superior to humankind" by being partly Undine.
Now, I know that some theories claim that the people that survived the sinking of Atlantis would go off and become the Maya and the Aztec - or at least join and influence them, but other than that, I don't know much about any theories in the direction.
So, as for the actual role in the RP:
The northernmost parts of Atlantis was said to be almost entire made up by large mountains (one of which was carved into a palace by Poseidon, with three large moats, growing greater with each, surrounding it, which humans later carved tunnels through and stuff to make it a huge city-like thing). For the sake of the RP and the "Bermuda Triangle" approach, though, these mountsins would need to have been in the southernmost parts of the island instead, which is such a small thing that I can easily see it being plausible. With the mountains in the southernmost part of the (huge) island, the Bermuda Triangle would be where the mountainous area of the island, and thus the city of Atlantis, would have been and the "deserted island" on which our characters will be stranded, will actually be the tip of one of the mountains, which stood tall enough to break through the surface of the sea that swallowed it. There will be several, smaller, islands around ours.
