Synonyms for Unsurmountable (same or very similar meaning)
WordNet sense 1 (incapable of being surmounted or excelled):
insuperableWordNet sense 2 (not capable of being surmounted or overcome):
insurmountableWordNet sense 3 (incapable of being surmounted or climbed):
unclimbableWordNet sense 4 (incapable of being passed):
impassable, unpassableFrom the ODE community, based on WordNetadd/edit