Final Fantasy XII was the main inspiration behind the Sand Sea, but as in Colonies everything has some explaining, so do the existance of the Sand Sea. The thing is that it exists in the middle of the only continent a huge landmass and because of that, there is no aquifers near it. This landmass is splitting in two right behind the location where the Sand Sea is located, making a giant gap there that after some milenia has passed and because of the dry and windy location, the terrain became the most fine sand, so fine that it looks like ash.

[quote=@Timeless]
 I'm... not quite sure what a High Forest would be...
[/quote]

The world where Colonies takes place has an atmosphere that is not ideal for humans, it is toxic, making it impossible for humans to breath and survive for more than 8-10 hours, in other hand, the High Forests, are forests formed in most by trees past a kilometer tall, their canopy are so tall and so full of leaves that the sun light never touches the ground below, the fauna and flora down there are almost everything with bioluminescence. The thing is, the atmosphere there is different, it is almost identical to the one in Earth, making living there for long periods possible, although very difficult, painfull and chalenging.