Synonyms of 'cataclysmic'
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If you describe an event or situation as calamitous, you mean it is very unfortunate or serious.
a calamitous air crash
Something that is catastrophic involves or causes a sudden terrible disaster.
A tidal wave caused catastrophic damage.
Thesaurus for cataclysmic from the Collins English Thesaurus
A cataclysmic event is one that changes a situation or society very greatly, especially in an unpleasant way.