(from ebookjapan):

Rieko Shirakawa, who lives alone with her brother Ryu Shirakawa, an ocean development researcher, is a high school senior in love with a rock 'n' roller. One day, Rieko visits her brother Ryu, who is holed up in the ocean research vessel Yamato, to deliver a package entrusted to her. When she opens the package, she finds her grandfather's diary, called the "Phantom Diary," in which he wrote, "I have discovered an underworld country!"

In the midst of the growing turmoil, the ship Yamato, diving at a high speed and crashing to the bottom of the sea. Rieko and her friends were prepared to die, but when they regained consciousness, they were confronted with a startling sight...!

This is an imaginative adventure fantasy based on the romantic theory of the Hollow Earth.