The story follows an orphaned young girl, Riko, who finds and befriends a humanoid robot Reg and descends with it into the titular "Abyss", that leads deep into the Earth, in hopes of finding her ...

The book was first published in 1999 with the title "Harbinger of History: Solemn Promises Made Half a Century Ago" (历史的先声——半个世纪前的庄严承诺) by Shantou University ...

It covers the period from when Riko descends into the Abyss with her robot companion Reg, reaching the second layer where they meet the White Whistle Ozen who reveals information about Riko's mother.

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