Properpolis is the capital city of HUM and the largest urban center in the Sylvian region. Situated in the original lands where HUM was founded, the name is drawn from the designation of the area as "HUM Proper" as HUM began to take on overseas populations and territories including Tautona, Goatwear Island and Sanctuary Island, ToeGANG, Etel Metsia, and Rhett. While the area had always aspired to be a city of some form, the new title of Properpolis began to be used when development kicked off and high-rise buildings such as the Aquascissors Memorial Governance Building began to be constructed.

The venerable city contains the world's first building (the HUM Great Hall, which originally served as GoatWhisperer's home) as well as several government buildings and offices, the world's first library (established before the Montrosian Archives to the west), the world's first post office, common resource storage for Hummites, an abstract art installation created by Ongstoes and Hongstoes, the original home of Mania_Man before he relocated to the building that would become the headquarters of the Prosperity League, and the world's largest vending machine built into the headquarters of Goatwear Industries, among numerous other features.