What is the name of the underworld in the Kalevala?

The underworld in the Kalevala is called Tuonela.

Tuonela is the realm of the dead in Finnish mythology, ruled by Tuoni and his wife Tuonetar. The name is closely connected with Tuoni, a figure associated with death, and related terms such as Manala and Vainajala can also mean the land of the dead. In some contexts, Tuonela can even refer to a grave or cemetery.

The Kalevala describes Tuonela as separated from the world of the living by the dark River of Tuoni. A ferryman or ferrym woman guides travelers across it, while the dead and dangerous supernatural guardians make escape difficult. The realm is not simply a place of punishment; it is a shadowy destination for the dead generally.

Väinämöinen enters Tuonela seeking secret knowledge and magical words. Tuonetar tries to trap him with enchanted sleep and the beer of oblivion, but he escapes by transforming into a serpent. Lemminkäinen also becomes linked to Tuonela when he is killed while pursuing its swan, before his mother restores him.