For my world I would need a really large swamp (or similar kind of wetland). It should be about 200km in diameter, which should be quite possible given a large enough flat terrain and enough rain.
The problem is that this environment needs to be completely isolated from the mainland: my small humanoids (around 70cm high), that have a technological level similar to that of natives of the Amazon basin, shouln't be able to reach the mainland or, if possible, shouldn't even know of it's existence.
Unfortunately, putting the swamp in the middle of the ocean like an island isn't an option, as the kind of plants able to grow in such saline environments are not suited to the environment I want to create.
This would lead to the need for a really large lake, which raises two questions:
- Is it even possible for a swamp to exist in the middle of such a large lake?
- I know It's possible for large lakes to have currents, winds and tidal movements, and even seasonal currents, like the Great Lakes; but could there be any conditions under which these currents and winds are so strong as to make it impossible for my civilization to cross this barrier? This could also help justify their knowledge of the existence of the mainland without any settlers there, and could lead to quite a bit of mythology I might get interested in.