From the Super Mario Wiki, the Mario encyclopedia
A Trampoline Block is a blue semisolid block with a diamond icon appearing in New Super Mario Bros. 2. It is at first invisible to the player, but then appears once the player steps on it and bounces them upwards at a fixed height before disappearing again.
The first group of Trampoline Blocks appears in World 2- near the end of the Boohemoth chase, where they are used to collect the third Star Coin. Several rows of Trampoline Blocks also appear in World Mushroom- in all three rooms between the second and third areas.
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