A pair of German tour guides take us on a vicarious stroll down Unter den Linden, the famed tree-lined boulevard running through the heart of Berlin, and describe what it reveals about their country's history. A British writer and avid "mudlarker" explains why low tide on the River Thames often brings pieces of that city's past to the surface. And an artist with a passion for street art tells us how she helped brighten up a couple of working-class neighborhoods in Rio de Janeiro.