Villages were very hard to build in the Swamp because it is hard to get around in there. So, some people built a small village in the trees which eventually became huge. They eventually adapted and controlled the swamp and are still there living with the Swamp Village.
This village was created when people from the Tree Village decided that if they had mastered the swamp, or were training to, that they should live in the swamp. So, the took all of the mud and water from the swamp and temporarily put it somewhere else. Then they built an entire village where the swamp was and then took the mud and water back to cover it up, they built it to be covered up. The entrances are five spots covered by a small pond of mud which only people of the swamp nation can temporarily move the mud to find a trapdoor to the village.