A very beautiful story with a meaningful message.
The characters are very interesting, and the way they faced their problems/traumas and emotions was portrayed realistically. The mystery of the tunnel and how they dealt with its existence and discovered how it worked was also very interesting and realistic.
I really enjoyed reading this manga, and the ending was satisfying.
The art is very good, with nothing to complain about.
I saw the movie first, this manga is superior to the movie and the story is more developed (although the light novel is the original source anyway, unfortunately it is not available in my language). I loved this manga and, of course, I highly recommend it.