Hellstone generates in the underworld. You can utilize the generated structures to get close to hellstone. Stay above the hellstone or keep a wall between it and your player character as each hellstone ore you mine will release lava. Try not to stand on the hellstone itself. Not only does it release lava it will burn you if you stand on it.
Hellstone really stands out between the ash of the underworld. Mine it with your pickaxe and stand clear of the lava that will spawn out of it.
Luckily, hellstone does not melt in lava, do not worry about catching it before the lava melts it.
While you are mining for hellstone keep an eye out for a hellforge.
These red hot furnaces will generate in any of the obsidian towers found in the underworld. Break it with your pickaxe and keep it safe until you can return home.
Finally you will need to mine some obsidian. Water burns out quickly in the underworld but you can still use a water bucket or an endless water bucket to turn a lot of the lava in the underworld into obsidian. Lava does generate in other deep reaches of your world so if the environment of the underworld is too hostile, search for a pool of lava close to the bottom of your world.
Obsidian will spawn when lava has contact with water. Drain water into lava or pour the water yourself. Mine some obsidian with your pickaxe.
With your hellforge, hellstone ore, and obsidian in tow you have all the necessary ingredients to make hellstone bars. Head back up to the surface and place your hellforge.
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