if it is writted,"Forty feet below, two million pounds are buried."
then perhaps it is simply forty feet below the surface?
Perhaps, the layers are code for the searching...... clay and youre there, coconut and youre too far....
Perhaps the treasure is ...7degrees out form the orignal tunnel?
Perhaps if the treaure is in someway related to the freemasons it will be spoken of within there writtings?
Perhaps the treasure can only be reached at set tied in some relation to the water tunnels?
.........
Sometimes when the puzzle is difficult and clouded, the answer is simple and clear.....just the mind confuses it up...

but, in saying all that.... there are alot of "perhaps"....... arrr mi hearties......where's the rum....