Ratlantis is an addon for Rats that re-implements some features that were part of the original mod since 3.0.0, in versions 7.0.0 and beyond.
Notably, the dimension of Ratlantis.
Ratlantis itself can be accessed through the creation of a Ratlantis Portal, which can be made by placing a rare token drop from a rat anywhere in the world.
Ratlantis is a lush, tropical land home to extremely dangerous and powerful mobs, like the Feral Ratlantean, Pirat, Ratbot and more, to name a few.
Using materials and drops from this dimension, one can craft extremely powerful Rat Upgrades like the Rat Upgrade: Nonbeliever and Rat Upgrade: Psychic. Powerful Ratlantean technology like the Rattling Gun and a Rat Biplane can also be made.
Ratlantis itself used to be a part of the base mod of Rats, but was split off into an addon due to numerous requests. More detailed information about Ratlantis, its bosses and more can be found on the wiki.