answer:Propane is a heavier-than-air gas, so any leakage would accumulate in the already underground structure. From there it would most easily continue to drain into floor drains before it could be dispersed enough to be rendered harmless. In the floor drainage piping system it would represent a massive, and building, continuous threat until something sparked it. In the outside, it’s far less hazardous, since the far larger and always-circulating air currents would easily disperse it before it found any natural or structural low points where it could accumulate.