Primeval Ankh-Morpork

Counts as Urborg, Tomb of Yawgmoth
Each land is a Swamp in addition to its other
land types.
Another legend claims that many centuries ago, the
Disc flooded. An ark was constructed, containing two
of every animal. When the accumulated dung of forty
days and nights was dropped over the side, they called
it Ankh-Morpork.