On the Bones of the Dead

Humanity appears to be the third species ever to inhabit Iodus. The planet was terraformed by a long lost civilization who left the surface as it was, and built massive underground caverns to serve as cisterns and algal growth pools for food and water. They also regulated the air until it suited their needs. They were, in turn, invaded by another species who took their great achievements for themselves. The invaders disappeared 20,000 years ago, and the translated scraps of their colony records seem to indicate they were attacked from underground by something left behind by the original terraformers. Whatever it was, it dragged off every single colonist, never to be seen again.
Human colonists have searched for years through the massive underground tunnels, but found no solution to the mystery. Most of Iodus's residents believe that the invaders got their just desserts, and since they didn't have to invade to settle Iodus, they have nothing to fear.

The terraforming machines of Iodus are the most advanced in known space...so much so that nobody can figure out how they work . Fortunately, they seem to automatically adjust air, temperature, and other environmental factors to make the world comfortable for the dominant species on the planet.