From the Super Mario Wiki
Sweatmore Peak is a volcano that Wario explored during the events of Wario: Master of Disguise. The majority of this area is spent traveling through the magma, but Wario needs the Sweatmore Hotpants to do so without taking damage. Many fish enemies swim in the magma, including the Battle Salmon from the Smithsnorian Museum. One enemy, the Snackeral, will take away Wario's Hotpants if he comes in contact with it. Wario will later need to find the Sweatmore Superhotpants in order to swim through the hotter magma.
It is revealed by Terrormisu at the end of the game that Sweatmore Peak is a nexus of dark power, and that's how she was able to reach out to Wario.
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