Crisis Core: Final Fantasy VII/Seven Wonders Expert

At the start of Chapter 8, talk to the little boy near the Water Tower in Nibelheim to begin the Seven Wonders Expert Side Quest.

Walkthrough
There are seven mysteries in Nibelheim to uncover, each time, you need to report back to the little boy in order to unlock the next one:
 * 1) Examine the Water Tower in Nibelheim to receive the Phoenix materia.
 * 2) Take a look at the Painting upstairs of the Inn, then go back to the boy, then again to the painting, then go downstairs and wait, the innkeeper will start to move (if he stands still, just go up and down the stairs repeatedly until he does so), follow him but not to close.
 * 3) You need to go to Mount Nibel, so leave the town by either exit, then north again. Follow the trail and then take a right at the fork (the 3 children there will give you tips about the monsters ahead), at the end, you will face 3 Bombs, you will need to defeat them before they explode.