Terraria Wiki → Underground
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Once the house is built, it's time to move on to excavation. There are many digging techniques, each with its own advantages and disadvantages. For example, a straight shaft going down will speed up the process of descending into the deeper layers.
Another popular method is exploring natural caves and tunnels. If a cave system ends in a dead end, exploring the walls with a lit torch often reveals another cave nearby. This method will yield more treasure, but will make it more difficult to escape to the surface.
Many players combine the two methods described.
Biome: Underground
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Enemies
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Blue slime
Red slime
Yellow slime
Pinky
Jellyfish
Giant worm
Drops from enemies
Gel
Glow stick
Fart pillow
Valuable resources
Copper
Iron
Silver
Precious stones
Glowing mushroom
Shimmering Root
Valuables in chests
Recovery Bracelet
The Magic Mirror
Angel statue
Cloud in a Bottle
Hermes boots
Enchanted Boomerang
Jester's arrows
Recommended equipment
Starting equipment
Recommended amount of life
The Dungeon is the first of the underground layers. It can be identified by the texture of the back wall (see image above).
This layer contains copper and iron ore. Silver and gold ore are also present, but are rarer and mostly found in the lower portion of the layer. Glowing mushrooms can occasionally be found in this layer.
Your main goal at this stage is to mine copper and iron. Before descending to the next layer (the caves), equip full iron armor and craft silver weapons. It's also recommended to find and collect two or three crystal hearts at this level, which increase your character's maximum health. To collect a heart, first smash it with a hammer, then use the small heart it drops.
Once you reach the caves, it's time to build the fourth house, which will house the demolitionist. Around this time, it's recommended to purchase a mining helmet (unless you've already found glowing mushrooms, in which case you can replace the helmet with potions of light).
Biome: Caves
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Enemies
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Black slime
Mother of slimes
Slime
Pinky
Giant worm
Skeleton
Bat
Jellyfish
Piranha
Dead miner
Tim
Drops from enemies
Gel
Fart pillow
Hook
Sorcerer's hat
Robot hat
Valuable resources
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Copper
Iron
Silver
Gold
Demonite
Precious stones
Glowing mushroom
Shimmering Root
Lava
Valuables in chests
Recovery Bracelet
The Magic Mirror
Angel statue
Cloud in a Bottle
Hermes boots
Enchanted Boomerang
Jester's arrows
Recommended equipment
Iron armor
Recommended amount of life
The caves are the second of the underground layers. They can be identified by the texture of the back wall (see picture above).
Copper, iron, gold, and silver ores are common in this layer. Glowing mushrooms are also regularly encountered.
Your main goal at this stage is to craft silver weapons and armor, then gold weapons and armor. Try to find a grappling hook and as many crystal hearts as possible (at least three or four).
It's dangerous for characters with poor equipment to be here. Don't even try until you've crafted iron weapons and armor. And regardless of your equipment, it's recommended to bring as many potions as possible, including several potions of Iron Skin.
Don't dig too deep at this stage. The next layer after the caves is the Underworld, one of the most challenging locations in the game. You may encounter monsters living there, and they're difficult to defeat even with a well-developed character. If you see fire imps, don't go any deeper. It's best to stop digging downwards as soon as you encounter the first pools of lava.
Source: gamer.ru
See also:
Meteorite bar (Part 1) crafting