Based on a plot from the Karelian-Finnish epic Kalevala rune 17 in which the main character Väinämöinen is swallowed by the giant Antero Vipunen.

Väinämöinen needs three magic words to complete his boat. He goes to Antero Vipunen an ancient giant and keeper of knowledge who has been sleeping deep underground for centuries.
When Väinämöinen tries to awaken the giant he accidentally slips and falls right into his open mouth. Once inside the hero refuses to give up. He builds a forge in Vipunen`s belly and begins furiously forging iron causing the giant excruciating pain. Tormented by the pain Vipunen is forced to chant his ancient incantations and reveal to Väinämöinen the three words of power he seeks so that he can finally leave his body.