Gin'iro no Olynsis

Summary

AD 3567. The human beings have lost their once civilization, and they are living on the desolated grounds. Although their lives are frightened by Gardener, a mysterious unmanned mobile weapon sent from the moon, strong boys and girls become Gardener hunters to earn daily bread. Furthermore, although the civilization was ruined, because of the high-tech maintenance free technologies, limited functions of cities are still working to support people's lives. Then, a boy's visit to a city opens the door of their destinies...

(Source: AniDB)