Gin'iro no Olynsis

AD 3567. Mankind has lost its once great civilization, now living in the ruins of the aftermath. But even though their lives are threatened by Gardeners — mysterious unmanned mobile weapons (of different ranks) sent from the Moon — brave teenagers have to become Gardener hunters to earn their living. And although human civilization at large lies in ruins due to high-tech maintenance free technology, some parts of the remaining cities still work in limited ways, supporting the humans living there.

One fateful day Tokito Aizawa meets Tea, awakening a power older and beyond that of the Gardeners, and inadvertently opening up the door to a possibly much brighter future for the remainder of mankind...

(Source: AniDB)