Once in the north of Iceland, you'll feel like you're on another planet. Instead of typical green hills, you'll see ancient mossy volcanoes, instead of ordinary beaches -- fairy-tale-ish black sandy patches of land nearby the sea. You'll see thermal waters gushing from the bowels of the earth and smoking hot whistling gases. Moreover, you'll notice that the waters of several waterfalls flow up instead of down, one of a thousand many other peculiarities of Iceland.
You'll notice the funnels of craters and pseudocraters protruding here and there and learn that cold glacial rivers flow through the valleys, but underground lakes are always warm.
A walk through the labyrinths of lava caves is a real quest; watching over the Northern Lights while lying in the hot bath is such a delightful experience that it's hard to describe -- something I recommend trying at least once in your lifetime.