Underground Jungle

The Underground Jungle is one of the more difficult areas you can find in the world of Terraria so far. It can be found close to surface or deep underground. The underground jungle always spawns opposite of the dungeon, so it is suggested that you should dig downwards on the other side of your map and continue digging horizontally to the opposite direction if you want to find the Underground Jungle.

The Underground Jungle is huge and takes a long time to fully explore. Shrines are exceptionally common in this environment, often containing chests with rare items or weapons that you can't find anywhere else. You can also gather materials that can be used to create rare items.

Hunting the Jungle
A more efficient way to find it is get a Depth Meter, travel to the farthest point on your map that is opposite your dungeon and depending on your map size, dig down to the appropriate depth, and start digging towards the opposite side. This can take a lot of time but it's guaranteed that you'll find one.

Keep in mind, that it might be deeper on your world, but these are general numbers.

Monsters inside the Underground Jungle

 * Man Eater
 * Hornets

Items in the Jungle

 * Anklet of the Wind
 * Cloud in a Bottle
 * Enchanted Boomerang
 * Feral Claws
 * Nature's Gift
 * Staff of Regrowth
 * Cobalt Armor (as drops from the Hornet and Man Eater)

Unique Material Drops in the Jungle

 * Jungle Rose
 * Stinger
 * Vine