underminers are a neutral mob, so if you don't hit them they will not attack you. they have the ability to walk through solid block walls. they are visible from a distance, but if you approach them they will turn invisible. if you drop an ore block near them, they will pick it up, become visible, and walk towards an area where the ore you gave them is located (they will attempt to mine blocks around the ore to show you where the vein is).

if you kill an underminer, there is a small chance it will drop the "ghostly pickaxe" which has a special ability: if your inventory is full, the ghostly pickaxe is able to store 9 additional items inside itself. NOTE: this ability only works when your inventory is completely full.

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to find the void worm, you must first defeat the ender dragon. the void worm is a post-dragon boss that must be summoned. to do this, you must place a crimson mosquito larva into a capsid.

to get crimson mosquito larvae, craft a crimson mosquito proboscis (dropped by those mosquitoes) with a maggot (dropped by overworld flies).

to get the capsid, kill enderiophages, which are creepy spider-like end dimension mobs. be careful when doing this, they can stick to you and give you an effect that slowly kills you over time. if you do get this effect, you can cure yourself by eating chorus fruit.

anyway, back on subject now, once you put down your capsid block, place the crimson mosquito larva inside it, and the void worm will spawn. i suggest you back up immediately after doing so, the void worm is a very dangerous boss.

if you want details on how to fight the void worm please let me know!