Pokemon Heart Gold and Soul Silver - Walkthrough - Tohjo Falls Pokemon Heart Gold and Soul Silver: Walkthrough - Tohjo Falls
This is a very straightforward cave. SURF up, and use WATERFALL to go up the waterfall.

Now make your way down the linear path of water, then go down the second waterfall.

Let's get an item quickly, so instead of SURFing to the exit, SURF to the right, and you will see a stairway. Go there. On the stairway, run all the way to the end, and grab the MOON STONE.

Now go back, SURF, and go to the exit to the cave.
This walkthrough was written by MKaykitkats, used with permission.