Hi there.
It’s very hard for me to remember this, but I’m sure it was about a boy and a slightly older girl. The world was a barren wasteland by the looks of it. I clearly remember a deserted desert (haha), and at one point they were walking in giant underground caves. The girl said “These caves were formed by *the giant worms/snakes*”. Can’t remember if they were walking through the skeleton of one or if they hear one coming but the boy gets freaked out.
I watched this movie 9-10 years ago, I honestly have no clue.
sounds a bit like ‘Nausicaä of the Valley of the Wind’
http://en.wikipedia.org/wiki/Nausica%C3%A4_of_the_Valley_of_the_Wind_%28film%29
and if you saw it 9 years ago, maybe it was during TCM’s Month of Miyazaki.
http://www.tcm.com/this-month/article/114160|0/Hayao-Miyazaki.html