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.
[[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)