Yami Sute Ana

"The Pit of Darkness"—an empty void that wipes anything thrown into it off the face of the Earth. Kasuga, a boy often taken advantage of by his peers for his diligent nature, happens upon a huge, bottomless pit during a field trip cleaning with his class. After accidentally dropping "something" inside, he realizes the pit possesses a strange power to... distort reality?!

(Source: MANGA Plus)