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
An efficient way to find an Underground Jungle is to 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.

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.