Indecipherable
Part of speech: adjective
Definition: Not decipherable; describing something that can not be decoded, read, understood or comprehended.
Incomprehensible
Part of speech: adjective
Definition: impossible or very difficult to understand
Example sentence: It is deeply shocking and incomprehensible to me that despite volumes of documentation and living witnesses who can attest to the horrors of the Holocaust, there are still those who would deny it.