It is pitch black, you are likely to be eaten by a grue.

I spent the entire summer of 1985 exploring the Zork I: The Great Under Ground Empire on the C64 that my parents had purchased from one of my neighbors. I can’t tell you how many nights were spent getting eaten by Grues and getting mugged by that pesky thief.

Needless to say I had a messy graph paper based map that I had created - although they looked nothing like these. Ah, the good old days!