Seven wonders

The seven wonders (七不思議, nana fushigi) of a certain place is a popular urban legend in Japanese culture. It consists of seven stories, mostly of paranormal nature, which revolves around the school and/or a location inside the school. It is commonly utilized as a central plot device for series with mystery elements, or used for a filler story in a series that belongs in other genres.

Seven in Japanese Culture
Like in many cultures around the world, the Japanese regard the number seven as a lucky number or otherwise important number in their culture. These beliefs, including the Seven Gods of Fortune and the belief in a person's seven reincarnations are often tied to religion, while others like celebrations at the seventh day of a person's birth and mourning at the seventh day and seventh week of a person's death is connected to life.

Origins
Most of the stories that consist a certain area's seven wonders are often rumors which are warped as it was passed from one person to another, developing into urban legends in their area if they persist. These rumors serve a variety of ways: to scare off other people or to add a certain interest to know more about the place among others.