There is a tunnel in the door, and at the end of the tunnel deep underground, there is a different kind of cave.
It is a space about the size of a football field, where the mountain wall is full of beautiful grass with little stars, so that even if the space is about two or three kilometers deep underground, there is no lack of light sources, just like a full moon in the morning and night.
In the center of the underground space is a deep pool, which almost occupies half of the underground space, leaving only one person wide passageway on both sides leading to the rear door. The pool is blue, just like a gem