[PTTG]

Urbanna is a distant alternate worldline. Here, wild places are no mana. The more densely populated an area is, the higher its mana level is, all the way to dangerous extremes.

This property has existed for a long time, and so the history of the earliest human settlements was radically different. The most ideal mana level for a stable society would generally be High Mana -- any higher, and magical disasters happen frequently, while lower levels mean that far fewer are able to cast spells. High Mana occurs at around 700-1000 people / square km, meaning that even a low tech settlement can surpass that limit without care.

The natives believe the effect to be tied to hearthstones and divine favor, though in fact it is a matter of where people sleep over a period of several weeks.