User blog comment:Lord Loss/Monster Hunter Theory... Blog/@comment-71.203.144.9-20121026025415/@comment-5043481-20121026172013

Could it be possible that the entire swamp was carved out of the landscape by a laha (large mud flow, ussually caused by volcanoes erupting into ice). This would make the entire swamp be covered in ash, and carve out channels. I think this mud is sometimes toxic, this could explain the poison.

Problem is with this is that laha's ussually set like concrete. Maybe the swamp was saved from this because of huge rainfall and underwater springs.