 The primary parameters we have to worry about in terms of exploiting metal resources is the grade of the deposit. How much of the element of interest is in the rock you're going to dig out? In terms of copper, for example, we can now mine rock that contains less than half a weight percent of copper compounds efficiently. The ability to dig out large amounts of material and eventually refine the amount of copper down to a pure metal is such that we can exploit resources of copper which would never have been possible, say, a hundred years ago. So we're down to very low grades concentration levels, but the other is the tonnage. How much is there of it? As a geologist working for a mining company, what you need to find out is how much is there per unit of volume or mass and how much is there? How many megatons is there? Then you might think, this is available resource. We can mine this. And of course there are lots of mines around the world where this approach has been undertaken and considered and they're active. The other thing, of course, you need to think about is the logistics of this. If you dig it out the ground, are you going to be able to get it to anywhere where you might process it, like a mill or a furnace or a smelter system? So these are the important criteria. Then you have to know whether is this accessible. So there's considerations here about where this can be done. But this is a non-renewable resource. We've got considerations about how we go about it. We've got better and better, more efficient at mining lower grade ores. And that just keeps getting better and better. Our ability to handle huge amounts, volumes of material out of a mine have increased dramatically. These days you probably know that the trucks driving around having been loaded up with iron ore in various open-cut mines in the Pilbara, they're all automated. They're run by computer systems. There's not a person driving the truck anymore, so the rate at which you can get the stuff out of the pit has increased dramatically. So there's a lot of technology gone on that's making the amount of copper and iron ore in the metal available to us increase with time. It's not a fixed amount. It's under economic issues that are controlling what we could consider as a resource.