Perhaps long ago a super advanced civilization decided that the planet didn't have enough land area and so built artificial continents.
Instead of bringing stone from stony asteroids and piling it up on the sea floor to build continents that were giant rock piles, for some reason the civilization imported a lot of metal from metallic asteroids and built gigantic metal pillars (presumably coated with anti corrosion substances) to support giant metal platforms that rose above sea level, and spread a thick layer of soil on top of the platforms to form artificial but somewhat natural looking islands and continents.
Or possibly the support pillars were made of advanced synthetic substances.
The story could be perhaps many millions of years later when the civilization that built the artificial continents is no longer involved with that planet, but there is a newer civilization the planet, either native or nonnative in origin.
And something happens to the support systems of the artificial continents for some reason and the support systems collapses. Parts of the platforms tilt a little but the width of the continental sized platforms is many, many times the total depths of the oceans so the continents mostly fall straight downwards. The continents displace water from beneath them until they get low enough that water starts to flow over the surfaces of the continents. Thus I expect that giant tsunamis will cross the oceans and strike the shores of any natural continents and islands of the planet, rushing far inland.
Of course the devastation caused by the giant tsunamis can be lessened by making only one small artificial island sink that way, or by making the artificial continents the only land on a planet with no natural continents or islands. Tsunamis crossing the deep open oceans should do little harm to lifeforms in the water, though they could stir up the sea bottom ooze greatly and kill all the creatures living on the sea bottom.