The vapour, where it is most dense, takes fire, like the blue flame of spirits, and these flaming clouds float to and fro, and yet do not burn the reeds.

The superstitious trace in them the forms of demons and winged fiery serpents, and say that white spectres haunt the margin of the marsh after dusk.

In a lesser degree, the same thing has taken place with other ancient cities.

It is true that there are not always swamps, but the sites are uninhabitable because of the emanations from the ruins.

Therefore they are avoided.
Even the spot where a single house has been known to have existed, is avoided by the hunters in the woods.
They say when they are stricken with ague or fever, that they must have unwittingly slept on the site of an ancient habitation.