Lets play a game, shall we?

What am I? II

I am smooth, round shiny and hard.

I have a wall and roof (of sorts) and I am mostly hollow.

I never travel more than a few inches, yet have covered a thousand miles or more in a single day.

I spend most of my time stationary, yet when I do move, I move at lighting speed.

I operate under extremes of temperature and pressure, yet very very rarely ever fail in my duties.

Some of you will recognize me in an instant, others have never seen me, nor in all likelihood, ever will.

What am I?

I wasn't quite sure about this at first, but it sort of grew on me...