The atlanteans all burbled and glubbed for air. 

“Hon Hon! Zer is mud everywheah!” 

“Oh no! My cigahwetts! Zey are in se swamp!”

Unfortunately for them, they were poor and not allowed to live. Womp womp. Atlantis disappeared off the map, its protected innards sinking into the mud releasing a violent gurgly pop as muck rushed to take its place in the bayou. 

Centuries later, a young man goes missing in the same spot, never to be found, but in reality he discovered the mud barrier, and promptly had his torso separated from the other part of his body. Not dead, just separated. Like when you leave a room and think what if object permanence is a lie but then you look back and your cat is still there chewing your TV remote and you realize nah, things can exist if I’m not with them. Atlantis has existed this whole time and we weren’t with it! Just as the halved boy’s upper body does still exist and is doing quite well for himself.