System's Twilight Hint Guide:

Underground River

I don't know what the specific transformations do.

• Removes the last letter and puts it at the beginning
• Changes all occurences of "Z" to "S", and vice versa
• Removes all letter after the last "E" in the word; if the last "E" is the second of a double "E", that gets removed too
• Changes all occurences of "O" to "E", and vice versa
• Doubles all vowels
• Divides an object (semantically) by ten
• Increments the first letter in every pair of doubled letters
• Adds an H to the beginning of a word