underwater cities might not be that bad, you it could be sort of like an advantage for underwater races that could make it more fun, and a way to make it more interesting, the pollution does happen, but, you could make quests or just an everyday task, to clean the water to keep your underwater city from becoming polluted, also, for flying races, the map could have upper area caves where the flyers could live, and also for the elves with good sight in the dark, underground caves, and for the humans or people who dont want any of the others, the city, there could be advantages and disadvantages to all of these, and this just makes choosing your race that much more interesting....