Answer:
It cannot be seen, it weighs nothing, but when put into a barrel, it makes it lighter.
I have one eye. See near and far. I hold the moments you treasure and the things that make you weep.
What lies in a tunnel of darkness. That can only attack when pulled back?
What has four wings but cannot fly and uses the wind but does not know why?
Alive without breath, As cold as death, Clad in mail never clinking, Never thirsty, ever drinking
You can always see it, but it's too far away to touch. Mountains rest on it, and at sea it surrounds you. What is it?