For those who have not completed games in the Xenoblade Chronicles series, this page contains spoilers regarding the plot. Discretion is advised.
The Cloud Sea is a location in Xenoblade Chronicles 2. It is an ocean of clouds overrunning the world of Alrest surrounding the World Tree, where Titans live. The Cloud Sea experiences tidal effects and periodically rises and falls. Some Titans such as Gormott and Mor Ardain undergo these tidal effects, sometimes allowing access to places hard to reach, while others such as Uraya and Indol fly over or under it and do not undergo them.
The World Tree stands in the center of the Cloud Sea. Surrounding the World Tree is the Great Void, an abyss where the Cloud Sea is not present and across which passage is nearly impossible.
It is revealed that the Cloud Sea was created by Klaus in order to restore life to the world in the aftermath of his experiment. The Cloud Sea contained particles that would eventually create the Titans. In the ending of Xenoblade Chronicles 2, Rex and company fly past the Cloud Sea on Azurda's back, revealing an endless ocean. The Titans then move to this newly revealed ocean and join together to create a gigantic landmass for all the inhabitants of Alrest to live on.