During the «Showdown! The Gloomy Cavern» quest step for the Carnival of Fireworks main questline in , you'll be tasked with opening a mysterious sealed door that leads to an even more mysterious cave.This cavern is found on the north side of the Firework Isle. You'll be led there by Marques Jr.
after receiving the mysterious sword from the stone, but you won't be able to get inside until you find all three Launch Tubes that keep it sealed.
These Launch Tubes, like the ones used to seal the doors in the other three caverns of the Firework Isle, are «hidden» across the island.
They're less hidden and more tucked away, but the map beside the door isn't especially precise, so you'll have to do some hunting to find them all. Many of them are found among or near groups of Esselings, so remember to be on your guard as you explore. If the Launch Tube hunt doesn't tickle your puzzle-solving fancy, check out the New Bloom Festival's Whim Tangram puzzles, which take both logic and patience to solve.
Once you reach the "" part of the Carnival of Fireworks main event quest, you'll get a waypoint marked on your map that takes you to a Warp Spire just outside the Gloomy Cavern.