When you drop an egg on the ground, it breaks. But when you drop a broken egg it doesn't reform a perfect shell. That's entropy — the simple fact that it requires energy inputs to maintain an open system, and a closed system only degrades over time. If there is to be Life, let alone civilization, there has to be some way of getting energy into the system. For the surface worlders, that's not even a problem: the Sun shines energy down on the surface all day. But for those who live in the dark realms, be it sewers, the ocean floor, or the classic dungeon complex, there has got to be a source of energy in the D&D world that just doesn't exist in ours. You don't see lush forests in marine trenches because the energy inputs just aren't there. But in D&D, neither the ocean floor not the underdar