insuperable (adjective) 

Something that is insuperable (in-SOO-puh-ruh-bul) cannot be surmounted or overcome. As used by J.R.R. Tolkien: “The original ‘Hobbit’ was never intended to have a sequel – Bilbo ‘remained very happy to the end of his days and those were extraordinarily long’: a sentence I find an almost insuperable obstacle to a satisfactory link.”