Nether

The Nether is the first dimension other than the Overworld which you'll enter. Like the Overworld, it's large on all 3 axes.

Layers
Layers are ordered from the highest elevation to the lowest.

The Top of the Nether
The Top of the Nether is very similar to The Top of the Overworld. It spans everywhere from 960 to 1216.

The Late Netherpause
The Late Netherpause contains very, very little, except for tiny chunks of rock. It spans everywhere from 896 to 960.

Dead Islands in the Sky
Dead Islands in the Sky is the Nether equivalent of the Skylands. They are very barren and very sparse. Dead, charred skytrees can be found on some islands. It spans everywhere from 640 to 896. All clouds are replaced with dense clouds of sulfur. Up here, the sky can be seen, revealing a large sun and a small moon.

The Early Netherpause
The Early Netherpause is a lot like The Early Pause, except, due to the mostly flat terrain, nothing generates here. It spans everywhere from 576 to 640.

The Surface
The Surface is the Nether equivalent of the Overworld. It's a barren wasteland. It spans everywhere from 320 to 576. There is no day/night cycle due to the fact that the sky cannot be seen. Dense clouds of sulfur block all light. Occasionally, it rains acid. Acid causes most mobs to take damage, similar to Endermen in the rain/snow.

Beneath the Surface
Beneath the Surface is a layer of the Nether that spans everywhere from 256 to 320. To get up here, soft bedrock must be pierced. There is not really much to say about this.

The Nether
The Nether layer of the Nether spans everywhere from 256 to 0. Its relative location in the world is more like the Deep Caverns than the Overworld. It's very similar to the original Nether, but it takes up twice as much space. It also has a new kind of fireproof tree.

The Deeper
The Deeper spans everywhere from 0 to -64. It contains very similar blocks to what the Nether does, but with almost no open spaces. A layer of soft bedrock must be pierced to enter here.

The Deepest
The Deepest is the deepest part of the Nether, spanning everywhere from -64 to -320. Random bits of lava can be found generating in place of blocks, in a similar pattern to ore veins. Much like The Bottom in the Overworld, around -310, lava generates in place of anything else. The bulk of the area is made up of a compressed form of Netherrack. Holes in the hard bedrock layer must be located to pass into The Deepest.