P D G H M P D G
P H M W M L Y W O
O F F 6 I M U S S G
P - Forgive me
Interesting... If you assume that each letter is the first letter of a word, then you could interpret that message as:
"Please Dear God Help Me Please Dear God"
"Please Help Me Will Me Live Your Will Only"
... (don't know about that third line)
"Please - Forgive Me"
Of course, it could mean something entirely different, but that was my first "impression" when I attempted to decipher it. I am assuming that each time a letter is used, it means the same word - this could be completely wrong. :)
Thanks for writing this up - it looks like it was a great adventure! I would be a little scared to stand under those "Acid Waste" pipes though! :)