The Wiki is the Underground equivalent for UTAUWikiTale. Unlike Undertale, the Wiki is not underground but the sky can only be seen in some locations. Each area, while following the layout of the Underground, have major changes related to the aesthetics and purpose.
The walls and floor are replaced with wiki articles and ink has replaced water. The sky appears black with a bright green square grid pattern appearing across it.
For more detailed information on the different parts of the wiki, look here.