Many years ago I toyed with a concept called "Caveworld" - the entire planet was, essentially, dried magma, riddled with holes, some of which run all the way through... But, deep inside it are the Nodes - each Node represents, in some way, the four classic Greek elements - Fire, Air, Water and Earth. The Earth nodes contain many precious minerals and are the reason why people settled on this world. The Fire Nodes could be tapped to provide all sorts of energy to power the colony. Air and Water nodes were essential for making the colon survivable.
And then there were the fourth Node types discovered, alternately called (depending on which group "claimed" them) Chaos, Mana or Core Nodes - those who could access them could work feats that could only be described as "magic" ... but sometimes they had flares that caused random things to happen - monsters to appear, gravity to reverse, etc.
Areas near Fire Nodes were in perpetual daylight (unless a Flare changed things), and the rest of the planet was underground with only artificial light available, as the surface was airless (except a few points near Air nodes, including the first one ever discovered) and frequently bathed in radiation from the planet's sun, radiation that going more than a meter inside the planet was completely blocked (except during some Flare events...)