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Synonyms for Unsurmountable (same or very similar meaning)

WordNet sense 1 (incapable of being surmounted or excelled):
insuperable

WordNet sense 2 (not capable of being surmounted or overcome):
insurmountable

WordNet sense 3 (incapable of being surmounted or climbed):
unclimbable

WordNet sense 4 (incapable of being passed):
impassable, unpassable


From the ODE community, based on WordNetadd/edit