Underground Jungle

The Underground Jungle is one of the more difficult areas you can find in the world of Terraria so far. 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
In the patches previously to 1.0.3, the efficient way to find the Underground Jungle was to get a Depth Meter, travel to the farthest point on your map that is opposite to your Dungeon and depending on your map size, dig down to the appropriate depth, and start digging towards the opposite side. This could take a lot of time but it was guaranteed that you would find one. This no longer works in worlds that have been created after the update 1.0.3, Underground Jungles are no longer on the edge of your map, they are directly under your Surface Jungle.

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

With the 1.0.4 update, you can now find the Underground Jungle by finding the Surface Jungle and digging down.

Another simple, efficient way of finding the Underground Jungle is to cross the first Corruption Zone that is not on the side of the map with a dungeon. Once crossed, dig downward on or near the border of the same Corruption Zone, and you should reach the Jungle.