Overview
A ecumenopolis is an urban structure that spans the entire surface of a planet or moon, converting the celestial body into a single, world-wide city. It is a hallmark of Type III civilizations, with every known civilization of this class possessing at least one ecumenopolis.
Ecumenopolization
Ecumenopolization refers to the colonization process by which a planet or moon is transformed into a ecumenopolis. This process is most commonly applied to S-Class, I-Class, and D-Class worlds, which typically lack native biospheres and thus pose fewer ethical or ecological concerns. Due to the extreme resource requirements and high cost — whether in material or currency, depending on the economic model of the civilization — ecumenopolization is not the default method of colonization. Terraforming is usually preferred unless the celestial body in question is unsuitable for such alteration.
The initial stages of ecumenopolization often involve constructing a dome or similar megastructure to provide an artificial atmosphere suitable for habitation. In some cases, this step is unnecessary if the species involved can survive in the native environment. The materials used in construction are sourced from the planet or moon itself or from neighboring bodies such as moons, planets, or asteroids within the same star system.
Design
Ecumenopoli are built in vertical strata, with hundreds of urban layers extending above the planetary surface. Each layer is separated by over a kilometer, and the largest ecumenopoli may contain more than 150 layers. The construction and maintenance of these vast cities rely on trillions of autonomous drones that perpetually repair and reinforce structural elements.
Natural oceans, if originally present, are typically either entirely depleted or rendered ecologically inert. In some cases, remnants of these bodies of water may persist deep below the surface layers, accessible only by intricate networks of suspended infrastructure.
Due to the near-total absence of plant life, atmospheric oxygen is produced artificially — primarily through electrolysis of water or the industrial cultivation of cyanobacteria. The oxygen demand of an ecumenopolis exceeds the total atmospheric oxygen of 21st-century Earth on an annual basis.
The energy demands of an ecumenopolis are immense. Power is required for transport, sustenance, climate control, and waste management. A critical engineering challenge is the dissipation of waste heat; without intervention, thermal accumulation would render the planet uninhabitable. To mitigate this, the surface is interlaced with a radiation network composed of high-surface-area metals designed to vent heat into space, functioning as a planetary-scale heatsink.
Population
Ecumenopoli support populations numbering in the trillions, with the most expansive reaching or exceeding a quadrillion residents. Their capacity for habitation is unparalleled due to their vertical and horizontal density.
Stratified Living Conditions
Life within an ecumenopolis is sharply stratified. The lowest levels suffer from extreme pollution and are often uninhabitable. These basal layers are characterized by environmental toxicity and structural decay. In contrast, the upper layers offer more favorable living conditions, with elevated property values and access to cleaner resources. The socioeconomic hierarchy of an ecumenopolis is typically reflected in its vertical zoning, with wealth and privilege concentrated in the highest strata.