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.
 * 4) You will need to open the safe in the Shinra Manor. In order to do so, you will need to find the four digits of the combination. Either try to find them by yourself (there is a note left of the Safe that will give you hints), or  use the solution:
 * 5) In the North-East corridor, look through the keyhole, count the books that are not in the shelves (count also those who might be on top of the shelves).
 * 6) In the east of the main entrance, look through the keyhole, count the monsters.
 * 7) In the West corridor, look through the keyhole, count the Banora dumbapples (purple) and the Banora Juice cans.
 * 8) In the west of the main entrance, look through the keyhole, count the chairs.
 * 9) You will need to find what's under the Shinra Manor, but only Sephiroth has the key, so go talk to him.

Reward
When you uncover the third mystery, the boy will give you Safety Bit. Opening the safe from the fourth mystery will give you the materia Vital Slash.