From the Super Mario Wiki
Thwomp Volcano (Japanese: ドッスン・ボルケーノ Dossun Volcano) is a currently inactive volcano in the northwest corner of the Mushroom Kingdom. The Mario Bros. came here in pursuit of Bowser during the events of Mario & Luigi: Partners in Time. Young Elvin Gadd used to live here prior to its annual eruption. Thwomp Caverns can be accessed by using Mr. Thwomp as an elevator, and deep inside the caverns, Bowser and Baby Bowser are fought.
After the Hydrogush 4000 rendered it inactive, the Mario Bros. journeyed inside after being confronted by Mr. Thwomp and Mrs. Thwomp. Mrs. Thwomp becomes outraged that the volcano became inactive and her husband loses his confidence. After the battle with Mrs. Thwomp, Mr. Thwomp agreed to help the Mario Bros. get a lift inside.
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