Terraria/Fridge

All the bosses after the Wall of Flesh are robotic versions of the previous bosses. Perhaps you're rebuilding them to become more powerful and more destructive.
We have: traps that are placed randomly into underground locations, mineshafts and dwellings that pop up in some pretty deep locations, a large, fleshy wall with two eyes and one mouth you summon by throwing a voodoo doll of the first human you meet, and somebody apparently rebuilding large monsters using machines. That means nothing good could have happened to those who took up underground.
Several of the hardmode items are crafted using souls. Think about it.
You summon the Wall of Flesh by chucking a voodoo doll of the guide into a pool of lava, and beating it releases the spirits of Light and Darkness. Just what IS the Wall of Flesh?

You can stop Corruption from spreading by planting sunflowers. In real life, folks living in Japan plant sunflowers to absorb radiation from the nuclear plant that was flooded in the tsunami.