A Thought Elevator in Kamine Island.
Thought Elevator is a term used by those affiliated with Code-R to designate structures linked with the Geass ability. Bartley Asprius speculates that some of the Emperor's recent invasions were done to gain control of the devices.
In appearance they are shown as an enormous vertical gateway from which bright light streams. Only two have been shown so far, on Kamine Island and another in the Geass Directorate's city and presumbly Area 18. There are presumably 8 or 9 of them around the world as shown in episode 17 of season 2 It is hinted that they were constructed by some long-forgotten ancient civilization.
Function
The true functionality of the thought elevator remains unknown; however it is known to work through a person's memories or thoughts as explained by Lelouch after discovering how they work, such as his escape from C.C.'s memory zone by the thought of wanting to help her.
They also serve as an entrance to the Sword of Akasha on several occasions. The Thought Elevator is also an essential element to use with the Sword of Akasha for the Ragnarök to occur.