The Atlantis dimension is an under water zone with cities.
The new blocks introduced is the Elder Sign, The Atlantis Gate, and Atlantian gold.
The elder sign is a bock that resembles a sign with a star on it, that is on the door to the forbidden chamber in the center of the atlantis dimension. It can only be broken with a daimond or obsidian sword. Once broken, Cthulhu will spawn.
the atlantis gate is a type of portal block. There are only 4 of them, and they are arranged in a sort of nether portal made of gold blocks. They can be acctivated with a Deep One Eye, which is crafted with an ender eye and 4 deep one scales.
It looks like a gold chisseled sand stone, and is as hard as bedrock.
Getting to atlantis
First you must craft a submarine.
Then you must go to the deepest part of the ocean and kill deep ones untill you have 16 deep one scales.
You then need 4 ender eyes to craft deep one eyes.
Deep One Eye:
NOTHING-ENDER EYE- NOTHING
You Then need to locate the atlantis Door, Which will have 4 atlantis gate blocks. Put an Deep One Eye into each, and then step through the portal.
You have to find the temple, R'lyeh, in the center of the atlantis dimension. Then you must break the elder sign with a diamond sword or higher. Once broken, cthulhu will spawn inside and destroy the temple. He will then proceed to destroy atlantis.