Wiktionary
untraversable
a. Cannot be traversed.
WordNet
untraversable
adj. impossible to traverse
Usage examples of "untraversable".
Odovacar himself, again a fugitive, sped across the plain south-eastward to Ravenna, compelled like so many Roman Emperors before him to shelter himself from the invader behind its untraversable network of rivers and canals.
Because he was not of their blood, White Crow could do things which they could not and visit worlds forbidden or untraversable by them.