Yucky World is a theme park for Grouches operated by Six Crabs.
On a particularly cheery day seen on Sesame Street in Episode 3997, a commercial for Yucky World airs on Oscar the Grouch's TV (making its way through his bad reception).
The narrator (Jerry Nelson) describes the park:
Admitting to having a terrible urge to go there someday, Oscar employs Maria in accessing the website at the Mail It Shop. Finding that the hotels are all booked up at the park, Oscar attempts to recreate the experience of going to the park at his trash can. After several failed attempts, his friends and neighbors go out of their way to serve him with the kind of Grouch cuisine available at the park, and simulate a roller coaster ride by locking Oscar in his trash can and thrashing him about (much to his delight).
Notes[]
Archived website screenshot.
A Tumblr post by "taperwolf" dated December 12, 2018 claims that when the episode aired, www.yuckyworld.org had not yet been registered. Fearing what the darker corners of the internet might attempt to do with a website endorsed by a childrens' show, taperwolf bought the domain and published a note for Sesame Workshop, offering to transfer it to their custody. [1]
The website is no longer live, but the earliest version available on the Internet Archive is a placeholder page displaying the graphic from the in-episode commercial and a short disclaimer explaining that "www.yuckyworld.org is a make-believe web-site." Clicking the graphic took the reader to a Sesame Street page on the PBS website. [2]