He starts off with saying that "there is a line of magma that is 15km long and 9 miles in length."

Do you know why he would mix metrics like that?

Edit: he's talking about a rectangle and his wording, to me, implied that he was using X/Y coordinates to talk about how big the rectangle is. Except that it sounded like he was mixing metrics. Other commenters suggest that he's just trying to help us dumb Americans. That could make sense too, but wasn't obvious in his wording to me. 15km ~= 9 miles, I should have done that math. =)