Start off by collecting the treasure lying around, if you want, then start looking for a Watery Blowhog and a pipe. Kill the Blowhog and go down the pipe, where you should see some fire geysers. Continue walking and you'll see a Fiery Bulblax. Use an ultra-bitter spray and your Reds. Snatch the key, watching the Pikmin carry it because Dweevils might appear: make sure they don't take it. Next, grab some more treasures if you like, and then go down the hole.
The high-strung, hungry Orange Bulborb has the key needed. Use your ultra-spicy spray. Sneaking up from behind is advisable, but most of the time it's in a small room, making that impossible. If you don't like its placement, reset the cave. Grab some treasure, since there should be lots of time (there is also a lot of treasure). Open the geyser and exit.
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