Mistmarsh

Occupying a huge bowl formed by the encircling Cairn Hills, the Mistmarsh is the source of two significant rivers - the Ery and the Neen, which flow together and then join the Selintan some distance below Greyhawk City. This area is a huntsman's paradise of fowl and other game.
The central reaches of the marsh, however, rarely encounter a human footstep. Of course, rumors abound-of the mad druid that lives there, the haunted cemetery of an ancient peoples lying at the heart of the marsh, the savage and reptilian predators slithering from the muck to claim helpless victims on moonless nights.
From its fringes the Mistmarsh is virtually invisible, for it gradually gives way around its entire perimeter to a wide belt of grassland, dry and smoothly devoid of any great trees.
As a traveler pushes through this grassy stretch, he gradually finds the ground squishing wetly underfoot and the grass around him growing thicker and higher. In summer, clouds of insects fill the air around him, creating at the very least an annoying nuisance. During a hot, wet summer the mosquitoes and biting flies are savage enough to discourage all but the most hardy from moving any farther into the Mistmarsh.
The ground grows swampier still, and soon each foot sinks calf-deep in sticky muck. Open patches of water begin to appear, never very large or deep but more and more common.
Then there is a clump of trees, and the weary traveler might think, "Dry land!" But instead these are the entangled groves of the mangaroo tree. This hardy growth takes root in soft marsh or even shallow open water, extending its tendrils ever outward and downward until it becomes a forest on stilts, as it were.
The mangaroo groves dot much of the Mistmarsh. The tangled trunks and soft ground underfoot create an effective barrier to most human passage. The only effective means of moving through the twisted mass of limbs is to climb up and down, over and through, snaking one's way through whatever passages present themselves. Needless to say, it is an exhaustive and discouraging mode of travel.