Create Category:Foo
Write [[Category:Foo]] and publish.
The description page shows that Category:Foo includes a subcategory Foo. Following the link brings you back to the same category. Opening the tree using the ►▼ bullets shows an infinite series of subcategories.

Example: https://el.wikipedia.org/wiki/%CE%9A%CE%B1%CF%84%CE%B7%CE%B3%CE%BF%CF%81%CE%AF%CE%B1:%CE%9F%CE%B9%CE%BA%CE%BF%CE%B3%CE%AD%CE%BD%CE%B5%CE%B9%CE%B1_%CE%9C%CE%B1%CF%81%CE%AC%CE%BD%CF%84%CE%B7

[[Category:Foo]] in Category:Foo page should display bold text instead of categorizing or a link as with other pages ([[Foo]] in page Foo does not create a link)