The adjective insurmountable describes some barrier that is incapable of being overcome. The Great Wall of China was meant to be an insurmountable deterrent to would-be invaders.
Although the adjective insurmountable can be used to describe an actual physical barrier, it most often refers to a less tangible challenge. "The runner was disheartened because her competitor had an insurmountable lead in the race." "Because of insurmountable difficulties, the school's fall party had to be canceled. The water heater burst, raining gallons of water down on the gym, and flooding it. Since it was not a swimming party, the school wasn't able to clean up the mess in time to host the party."
