Cenote

This world, somewhat older than the Earth,  had once supported a biosphere mostly consisting of calcareous algae. The original atmosphere, predominantly carbon dioxide, was removed over a period of more than a billion years and many sedimentary layers of limestone were deposited. Additionally large amounts of reduced carbon in the form of keragen was deposited in the sedimentary rocks, leading to a gradual decrease of carbon dioxide in the planet's atmosphere. But tectonic activity eventually shut down, and the carbon dioxide level in the atmosphere fell too low to support photosynthesis. The ecology of the planet collapsed when the phytocoral died back, and the planet was almost lifeless when the first explorers arrived from the Solar Dominion.

Despite having a minimal biosphere, the atmosphere had a respectable oxygen component of 10%. Over time this would be absorbed by the crust, leaving only nitrogen and argon. Salty seas covered just less than a third of the planet, but the atmosphere held enough water vapour to allow rainfall. Some of this precipitation remained on the planetary surface, forming lakes and wide rivers; but much of the water found its way underground into the innumerable limestone cave systems in the extensive karst landscapes.

The Solar Dominion long distance linelayer Yucatan arrived here in 6407 with a microwormhole and a small cargo of digitised colonists, who were given new bodies on arrival. These pioneers were fascinated by the cave system and most requested that they be incarnated as water breathing humans. The planet itself was given the name Cenote, in reference to a much smaller cave system described in records of old Earth. Only a few remained on the surface, working to increase the oxygen level and constructing small cities carved into the karst for the next wave of colonists who would arrive once the wormhole opened. But many of the Cenotean colonists dedicated themselves to exploring and mapping the underground world of the caves.

Tens of thousands of cubic kilometres of cave were surveyed, much of it by the explorers themselves, but large parts of the underground world remained inaccessible. The Cenoteans developed a number of different remote exploration devices which they could operate remotely in restricted space, including microscopic threadbots which could enter minute fissures. With such devices they discovered many large and deep caverns, often filled with the salt water of the oceans with a layer of fresh water on top separated by a distinct halocline.

The giant calcite crystals in Spukil Cave are among the largest in the Terragen Sphere

Elsewhere they found dry caves with giant crystalline deposits, or massive stalactites deposited over millions of years. As the population of Cenote increased, new, artificial caverns were excavated, some filled with salt and or sweet water, and some dry to house the surface dwellers. Large resources of oil and gas were also found, all remnants of the ancient and largely vanished biosphere. Some of this oil was used to make polymer products for export- oil- based plastics were quite unusual in the Solar Dominion, and such products had a certain novelty value.

Some of the caves were so difficult to gain access to that they were know only to a few; in time these secret caves became home to a society of hiders, the Hypogeans, who used geothermal energy and mined hydrocarbons to survive without interacting with the outside world. These hiders remained undetected for nearly a thousand years, although they were the cause of many legends and tales. Often a fleet of threadbots would be dispatched to search for the truth behind these legends, but the Hypogeans used a number of largely autonomous countersurveillance mechanisms to deactivate these devices. When a curious transapient Dominion Luminary from the local Prefecture joined the search, and employed microscopic explorer devices resembling oolite grains, these defences were finally breached, and contact was made with the isolated society within this labyrinthine world.

  More recently the Hypogeans have largely departed to join other hidden clans in the outskirts of various nearby systems; but some few remain, either as willing hosts to curious and adventurous tourists, or (it is rumoured) delving ever deeper into the bedrock of the planet.