This is an area of the inner Amazon Rainforest in Brazil.

Made in WorldPainter with Lentebriesje's Tree Repository, this is as close as you can get with a realistic rainforest in Minecraft. This massive forest has two mighty rivers: Rio Negro in the North, and Rio Solimões in the South. But, there's many small rivers flowing into the big ones.

Update 2.2 (Diversity Update): More plant species, ground covered in dirt, denser forests and vegetation and harder to get food

Tree leaves may not decay when cut down. Watch out for monsters though, even during the day. This adds realism, because there could be dangerous animals in the real world rainforests.
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In multiplayer this map could be used for survival games and factions.

This is the volcanic Nordic island known as Iceland, made in Minecraft with scale 1:36, which means the map dimensions is 16300x12300 blocks! This is a quite difficult survival map. If you can't find a forest on this volcanic rock before the sun goes down, you're going to die. This map is made for servers, because it's too large to be a SSP map. Made using WorldPainter with height maps.
Major update 1.3: More realistic trees by Lentebriesje. Contains five different types of forests.