It sank after the War of Wrath, but how? Did sections of the continent crumble as the two sides fought? Did the dragons make great holes in the earth when they fell from the sky? And where did all the survivors find land?
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I found a possible explanation.
https://middle-earth.xenite.org/how-was-beleriand-destroyed-in-the-war-of-wrath/
You answered your own question :)!