Mount Dys

Probably the biggest of the underground complexes across all of Ethereia, Mount Dys was originally an epic mining operation for rare metals in one of the greater mountain complexes at the times prior to major terraforming.
It's vast network of tunnels, personnel living and transport hubs as well as it's many midway refineries and supply stations made for a convenient basis for construction of multiple Planetary Drives - the gargantuan photon beam thrusters built for accelerating and steering the planet - that was built in ancient times across Ethereia for the purpose of Wandering.

Underneath these Planetary Drives was usually established giant shielded underground cities for the purpose of having easy access to engineering teams in cases of malfunction or general upkeep against wear and tear as well as ample personnel for the fleet of surface mining and logistics vehicles to provide reliable supply of fuel and other necessary materials for The Initial Era of running the drives.

Nowadays a major portion of the Planetary Drives have been decomissioned and repurposed for power generation, factories, surface habitation, research facilities, and in a few cases space elevators. Simultaneously their accompanying shielded cities have either become mostly uninhabited or developed into expansive underground complexes by way of merging as they grew into eachother. Mount Dys is one of few such series of closely located installations that has managed sustained growth after The Initial Era, and on top of that, is one of few such complexes that still has most of it's accompanying Planetary Drives in a vastly modified form but still functional and occassionally activated for trajectory adjustment of the planet.

It is therefore also one of the most high-tech and industry-heavy hubs of human activity and is continuosly managed by a highly technocratic and to some degree mysterious form of government as another remnant of the ancient times underpinning it's existence. Famously, this Government of Mount Dys is known as the main controlling body of it's similarly ancient bank First General Bank of Mount Dys, to which the Government of Mount Dys and in some way the complex as a whole is one of the largest contributors among Universal Income Suppliers.

The surface metropolitan area, while popular and experiencing consistent net-positive immigration flow, is sparsely populated compared to other underground complexes due to the wide-spanning exclusion zones around the still technically active Planetary Drives, and only ranks as the 11th most populous surface city. All estimates still project, however, that a significantly higher population lives underground still, although the true population count of most underground complexes is unknown and difficult to gauge, so the real population size and ranking of Mount Dys is difficult to know for sure, maybe even to it's secretive government.