okay a brief walk through:
1: enter each of the fallowing caverns:
top most
bottom most(surf needed)
both of the left most(need to go through union cave)
2: pass through the first wall and interact with the second
each time this will warp you to the main cave
just turn around and take the ladder
3:after visiting all 4 caves reenter the main cave
4: pass through the gap in the wall next to the ladder
5: go past the hiker and through the next gap
6: find the cave like entrance in the back wall and go through(this and everything past here is not in nintendo's version it is original to pokemon 3d so the maps
Myrkur posted will not be accurate)
7: go to your right and find the gap in the wall and go through
8: head to the left and talk to the third statue you pass
9: continue on in the same direction and pass through the other gap(this will warp you)
10: follow the path of gaps in the walls to reach a large cavern with the old man
11: talk to the old man to begin the event
12: retrace your steps to where you warped in step 9 there will be a cave entrance along one of the walls this will warp you back to step 6
13: done