The Underworld

Deep under the Cavern Layer, The Underworld begins. The spawning of Fire Imps and Bone Serpents and a change in background texture marks the beginning of The Underworld. It is made of Ash Blocks, lava, and Hellstone ore. There are also Hellstone Brick buildings scattered across the landscape, which sometimes contain Hellforges.

The Underworld is the most challenging environment so far, giving you access to Hellstone, which can be smelted with Obsidian in a Hellforge to create a Hellstone bar, the main ingredient of some of the the strongest armors, weapons and tools. A nice tip is to find a large pond of water in a cave above, and dig down until the water flows down into The Underworld. If you have enough water, you can easily collect Obsidian without worrying about falling into lava. This technique is called Earthwelling.

It's worth noting that, if you move down as much as you possibly can, the camera will stop aligning with the player.

Tips and Tricks
You can build a 2 by 2 tunnel down to the bottom of the world which then reduces the monster spawn rates, especially for the annoying Fire Imps. However, this also reduces the chances of Bone Serpents, though if they do attack you try to make them run off the entire map/screen and they'll disappear. This gives you the chance to mine as much Hellstone as you want, but be warned! You need to be careful as more you expand your cave; the larger the cave, the higher chances of Fire Imps.