Made in Abyss

An orphaned girl named Riko lives in the Belchero Orphanage in the town of Orth, which surrounds a strange, giant hole descending deep into the earth, known as the Abyss. The Abyss harbors artifacts and remnants of civilizations long gone, and is, therefore, a popular hunting spot for so-called Cave Raiders, who undertake arduous and dangerous descents into the mist-filled pit to recover whatever relics they can find. Some legendary Cave Raiders earn the title of White Whistles, one of them being Riko's mother, Lyza, who is presumed dead after taking a "last descent" into the Abyss. Riko's longing in life is to follow in her mother's footsteps and become a White Whistle. One day, she discovers a robot resembling a human boy in the first layer of the Abyss and names him Reg. Riko, determined to find her mother, bids farewell to her friends and secretly departs into the Abyss with Reg as her companion. (Source: Wikipedia)