As you probably know every respectable RPG needs an underground complex below a city... Usually it involves high gov spending on spacious severs full of mutated creatures... such constructions are usually hard to justify to taxpayers :D OK, more seriously, the setting: 1) the city is located on a cool, habitable exoplanet, near a big river, surrounded by semi-desserts (think like Nile but temperature is usually somewhere around 10C-20C) 2) There was general idea to place the city somewhere in lower part of that river. The specific choice was made based on selecting the most reasonable site - some natural resources and suitable place for construction. 3) Technology is not much better than early XXIst century. 4) Restrictions: -the planet is very poor in fossil fuels, so they are automatically out -there is no point in mining any luxury stuff at that moment like ex. gold (yes, I know a bit used in electronics, but still don't fit) -under perfect conditions the mining would start with strip mining and later there would be some shafts -the rock should be solid enough that such mining would not preclude in any way subsequent construction of low skyscrapers -the substance should be something essential in huge quantities for survival of industrial civilization, like ex. iron or aluminium 5) Intended history: -first a mining town with industry processing that ore; -evolving in to big city, where mining would become marginal activity; -turning empty shafts in to an underground district of city, that just in case could be used as nuclear shelter and is connected with outside world with lifts and metro. Which geological formation and ore would you advice? At best solution would be good at mining, reasonable source of construction material for whole city and simultaneously look as advantageous combination and NOT like a miracle.