It's a PITA, and the only way I got it was to break down and use a guide, as
I would never have spent the time figuring it out....Anyway, it's part of
the waterfall puzzle and I'll list the instructions out of the guide I
used......

** WATERFALL PUZZLE **

To start things off, you need to go back the way you came by enter the path
on
the southeast side.  I have drawn up some maps with the correct path
outlined
that should be taken in order to stop the waterfalls.

Copy and paste the url into your browser to see the maps.

Waterfall Puzzle Path 1 - http://tinyurl.com/y7rkwy

Waterfall Puzzle Path 2 - http://tinyurl.com/t4xqt

Each time you enter a new area the message "The course of the waterfall
seems
to have changed" should be displayed across the screen to let you know that
you
took the correct path.  The paths that you make do not need to be precise,
but
you MUST enter and exit from the correct pathways when entering the bottom
(Mirror of the Soul), middle (Falls of Time), and top (Destiny's March)
areas.
You can think of both maps as one long path, but I thought it would be best
to
have two to avoid confusion.  You'll get that previous mentioned message
three
times as you enter the different areas, then on the final time you'll
receive a
message that says "You hear a door open in the distance".  This means the
middle path in the Falls of Time area is no longer blocked by a waterfall
and
that the Pilgrim's Door is now unlocked.  Walk toward the middle area and
open
the treasure chest on the other side of the small bridge around the stone
structure for a chance at either an ++ELIXER++ or a ++DARK MATTER++.  There
is
also another chest off to the east that contains either an ++ELIXER++ or a
++KIKU-ICHIMONJI++ Katana.  To the south, open the first Pilgrim's Door to
find
a chest containing either the ++KOGA BLADE++ or the ++IGA BLADE++ Ninja
Swords.
The only other way to get an Iga Blade is through a drop by Bogey's in the
Zertinan Caverns, so you may want to save your game after solving the puzzle
and open the chest until you get that weapon if getting every weapon in the
game is important to you since none of these chests will respawn.  The Koga
Blade can be achieved through a side quest.  Keep in mind that the chests
are
random, so if they don't all three appear then reload your game from the
previous save point (after saving when the puzzle is solved of course).  As
long as you haven't opened one, then they will appear eventually.

Good luck...

IYM