Urasekai Picnic

The novel is set in the modern world, but there exist doors that can take people to the "other side," where strange urban legends and internet stories prove to be true. College student Sorawo Kamikoshi finds herself exploring one of these doors, and almost dies on the other side, when she is rescued by another girl named Toriko Nishina. Toriko is mysterious but skilled with a gun, and she seems to be searching for someone else on the other side. Together, both girls explore these doors to the other side. (Source: ANN)