The marshy districts of Sologne, Brenne, Landes and Dombes still contain large undrained tracts.
The streets as originally laid out were wide and spacious, but being unpaved and undrained they were no better than mud tracks diversified by piles of garbage and foul-smelling stagnant pools.
There are undrained, swampy districts in Campeche, in the vicinity of the Terminos Lagoon, where malarial diseases are prevalent, and the same conditions prevail along the coast where mangrove swamps are found.
As a whole the system forms the watershed between rivers flowing to the Indian Ocean - the Indus and its tributaries, Brahmaputra and its tributaries, and Salweenand the streams flowing into the undrained salt lakes to the north.
The old town is the upper or northern part, and is inhabited by the poorer classes, its streets being badly paved, crooked, undrained, dirty and pestilential.