Pen Drawings From Youtube

I've been looking on youtube at some ball point pen drawings, i stumbled across these and my mind was blown, in shock to how such a smooth textured drawing can be achieved using a ball point pen. None of these drawings show clear pen lines or evidence of pen strokes to seem like a drawing what so ever but just look like a photograph. 

This is another area i want to look into and put my hand to, using pen isn't something i've ever tried before because i never thought such realistic images were possible, but it's yet another challenge i wish to defeat and eventually produce some drawings of my own which you have to look at twice to believe them and question their medium.