There is a structure in the middle of the Underworld in Terraria.It can happen frequently and hold desirable loot.
Step 1: It is necessary to find protection.
Necro/Crimson, Gold, and Silver are good armor.You should have your strongest weapons.If you have an item that grants immunity to fire or lava, it is a good idea.These are useful if you have the Lava Waders.You most likely will be harmed by Fire Imps if you bring several Potions of healing.Get some wings if you don't want to sink into the lava.The only way to get Shadow Chests is by using a shadow key.There are gold chests in the dungeon.You can make a quick escape if you have a Magic Mirror or a Potion of Recall.
Step 2: A pickaxe is a good purchase.
If you're going to the Underworld for the first time, it's a good idea to have a Deathbringer Pickaxe as you may be digging for a long time before you get there.If you've been to the Underworld before you should use a pick axe.This is important to your survival and will maximize your loot.
Step 3: Straight down, dig a two block tunnel.
caverns provide a quicker way down, so be sure to look for them.After a couple of minutes of digging, you will see a wall of rock with many glowing lava veins, if you do this correctly.You have arrived in the Underworld.
Step 4: You have to work until you get to the ruined house.
After traversing across the terrain for a little bit, you'll likely find one made up of Hellstone or Obsidian Bricks.It's important to check for loot.Those Fire Imps and Hellbats are dangerous.
Step 5: Thank you!
You have found and taken items from a ruined house.Before you can get your Potion of Recall or Magic Mirror, you need to find some more loot.