Mythical Island Destructions, from Least to Most Hardcore

Atlantis
No. Weak. Everyone knows about it. Sunk by the Greek gods in an earthquake because the people were greedy and kind of boring. Not very hardcore.

Cantre'r Gwaelod
A Welsh island that flooded when either (a) the well-maiden Mererid let the well overflow or (b) the prince Seithenyn got drunk and forgot to check the sea-embankments, depending on which version you favor. Either way, it sunk because someone forgot to do their chores. Yawn.

Lemuria
Started out as a theoretical continent in the 1800s, proposed by a zoologist named Phil because he couldn’t understand why there were lemur fossils in Madagascar and India but not in Africa, so naturally he jumped straight to “secret sunken continent full of lemurs.” Discredited when people started doing Science for the first time, but then occultists took hold of the idea and decided Lemuria was the cradle of human civilization and/or the source of all esoteric knowledge. Often equated with Mu (see below), so not very original and thus only mildly hardcore.

Lyonesse
Tennyson said it’ll be the site of the final battle between Arthur and Mordred, fated to sink beneath the waves. Though it didn’t actually sink beneath the waves; it’s more of a future thing. But other legends say God drowned it in 1099 AD because the inhabitants committed some unknown crime so nasty that He couldn’t bear to look at it anymore. So, decently hardcore.

Mu
Home of the advanced Naacal society, who supposedly started civilization itself in Egypt, India, Babylon, and Mesoamerica. Got busted up by volcanoes, which is fairly hardcore.

Buyan
An island in Slavic mythology that disappears and reappears constantly. Bonus points for surviving its own destruction over and over, and major bonus points for being the spot where Koschei the Deathless kept his soul hidden in a needle, which was inside an egg, which was inside a duck, which was inside a rabbit, which was inside an iron chest, which was inside a freaking oak tree.

Ys
Destroyed because of too many orgies. Which easily puts Ys in the top two. It was a city built below sea level, and it fell when Dahut, daughter of the king, gave the dike key to a red knight who turned out to be the actual devil and flooded the city. Dahut turned into a morgen or mermaid in the aftermath, obviously. Pretty hardcore.

Numenor
Yes. Drowned by Middle-earth’s Big God because Sauron convinced the inhabitants to start worshiping the devil so hard that they went crazy with punk rock and human sacrifice and sailed to Heaven to fight the gods. But the gods weren’t allowed to mess with humans directly, so they called on Big God to help them out, and Big God changed the shape of the world, which had formerly been flat, into a globe, so that Heaven wouldn’t be sailable-to anymore, and buried the attacking navy in dirt, and sank Numenor under a huge wave forever, killing everyone except the non-devil-worshipers, who had sailed away earlier, wisely. Excessively metal.