From the Super Mario Wiki
Blargg's Boiler is a subterranean cavern found somewhere between the Bone Dragon Pit and the pipelines in Yoshi's Story. It is completely flooded with lava, and so the only way to cross is by traveling atop the bridges of giant bones.
As the page turned an ocean of boiling hot lava bubbled out. If the Yoshis didn’t cross, they would never find the super happy tree. Blargg and the lava were scary. The Yoshis bravely fought their way through.
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