insurmountable

Morrel expected Villefort would be dejected; he found him as he had found him six weeks before, calm, firm, and full of that glacial politeness, that most insurmountable barrier which separates the well-bred from the vulgar man.

For decades, athletes and experts alike considered the four-minute mile an insurmountable barrier to human athletic endurance.

Without the foundational knowledge acquired through these localized observations, implementing effective medical interventions in the wild would be an insurmountable endeavor.

Sacagawea provided an extraordinary synthesis of linguistic dexterity, geographical intuition, and diplomatic acumen that repeatedly rescued the explorers from seemingly insurmountable predicaments.