The Elevator Fushigi na Elevator

One morning, a boy dashes out the door of his apartment carrying a school bag on his back. He jumps into the building elevator as on any other day; but today, it won’t stop and just keeps on going down.When it finally stops with a jolt and the door opens, the boy finds himself in a world deep under the ground. There his adventure begins.

(Source: yamamura-animation.jp)