Tuonela Planitia was discovered alongside Triton's other surface features by the Voyager 2 spacecraft during its flyby of the Neptune system on 25 August 1989.

[3] The name Tuonela Planitia was approved by the International Astronomical Union (IAU) in 1991; the name originates from the underworld realm of the dead from Finnish mythology.

Tuonela Planitia cuts into the older cantaloupe terrain, indicating that it (along with other walled plains) is one of the youngest features on Triton's surface.

[4]: 921, 940  The shoreline-like profile of Tuonela Planitia's walls indicate that they may have been eroded by fluid sitting within the basins for extended periods of time, and the surrounding smooth terrain may represent sites of viscous cryolava flows or cryoclastic deposits.

In particular, Tuonela Planitia's eastern wall is host to a system of short, dense, narrow channels that may have been carved by glacial activity.