Zebes

Zebes is a major planet featured in the Metroid series of games. It first appeared in the NES classic Metroid], though it, along with anything living there, was later destroyed. It is a large planet of 4.8 trillion teratons that features underground seas and a large cavernous crust.

While the date that Zebes was discovered is unknown, it has been confirmed that it was found by accident while a group headed by a guy, presumably known as Hakro Zebeth, was going through a galactic trade route. Interested, he went onto this uncharted planet, where he became lost. He eventually sent out a distress call, though all hope was lost and the crew were never heard from again. It should be noted that this has been the case for multiple other teams as well, who all seemingly died on the planet.

The planet
One of Zebes' most notable feature are the underground caverns that are found throughout the planet that lead underground to an extent where no human being can survive due to the tremendous heat that's been radiated by the planet's core. However, some of the upper caverns are home to a majority of the planet's species.