Luna-tan: 1-man Nen no Himitsu
There was a prospering civilization thanks to giants called Luna who once brought the energy known as Mana. However, a giant tsunami struck the world and destroyed the civilization. The story takes place underground 10,000 years after the tsunami hit. The main characters are Mokugyo, a fish named with plant life growing from his back, and a talkative creature named Toriiruka. The lonely, needle-covered animal named Harry and Yona, a descendant of the ancients entrusted with with the rebirth of civilization, join the group. Mokugyo and Toriiruka may be able to safely return above ground.
[Source: Anime News Network]