Some natural melon blocks explode when broken by a player. The explosion is massive (explosion power of 28) and very deadly.
If you're too scared of blowing up your jungle, you can turn all explosive melons off with the explosiveMelons
gamerule.
They drop nothing when broken.
They're also craftable if you're feeling evil. Crafted like a normal melon block but with one of the melon slices replaced by a gunpowder.
Their weakness
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They produce smoke if a player is sneaking nearby. Thus, you can find the Explosive Melon and dispose of it appropriately (for example, another, smaller, explosion).
Every player gets one Warning the first time they break an Explosive Melon in the world. This Warning un-breaks the Explosive Melon, temporarily gives you Creative Shock, and displays a big scary message on the screen. So you have one chance before being eviscerated.
An explosive melon in its natural habitat.
Regular non-explosive melons in their natural habitat.
Crater from a melon explosion.
An explosive melon producing smoke when sneaking.
Warning message when breaking for the first time.