Cavarn

Seen from orbit, Cavarn is a dead planet: a grey crust, cracked, without water or vegetation. It was an observational error that lasted centuries. The life of Cavarn is on the inside.

The world below

Cavarn is not hollow: it is hollowed. Within the thickness of its crust open caverns so vast that the horizon is lost in them — chambers hundreds of kilometres across, with ceilings too high to be seen, joined by galleries as wide as rivers. The ground there is a true ground, weight falls as it falls anywhere, and water runs downhill. There are lakes down there, and forests, and wind. There are even clouds.

The crystal sun

What lights this underground world are crystals set into the ceilings. They brighten and fade slowly, following a steady cycle of eighteen hours — a true day and a true night, made of stone. No one knows whether this is a natural phenomenon or somebody’s work.