I found this fruit in the South of France, but I have no Idea what it is. It's a green, hard fruit. I tried to cut it to see the inside (on the following pictures).
It fell from a big tree, but I don't remember much information about it.
Could it be a Ceiba speciosa pod? We have a lot of Silk Floss trees near where I live and they grow these types of pods. When they are fully developed, they burst open and are filled with cotton-like floss and dark seeds. I have never seen an unripe one opened up, but it definitely looks like the ones on the trees. The trees themselves are easily identifiable as well. They are large with greenish trunks that have thorns growing along the surface. Also, if you had previously seen the tree with large pink-purple flowers, that would be a good indication.