How can I save the world from both good and evil corruption?

Question

After the Wall of Flesh, the hallow appears and corruption spreads even faster.

• Corruption spreads through everything
  • Except Hallow
• Hallow spreads through everything
  • Except mud (jungle)
  • Except synthetic blocks like gray-brick
• The results of Hallow and Corruption meeting are unpredictable

How can I stop them both permanently and effectively from taking over the world, considering both the top layer and underground?

(Eg, simply placing gray brick on top of the surface won't stop the underground spread?)

And, is it worth the effort?

Answer

It might be possible in theory, but it would take an incredible amount of effort and is, in my opinion, not worth it. If you remove all corruption and hallow, you'll be removing the sources for quite a few of the late-game materials.

If you're really hell-bent on it though, your best bet would be to, as soon as it appears, surround it with 4-wide tunnels/shafts. Corruption/Hallow can jump up to 3 blocks, so a 4-wide tunnel will prevent it from spreading past it. Then you can use purification/vile powder to clear it, although that will be quite tough as well, as you'll have to tunnel a grid through it to ensure you get all of it. You might want to infect a few of your flying islands with corruption and/or hallow first, though, so you still have some access to the materials.

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