S1, E2: Why are London's black cabs so oddly shaped? How did the city's first toilets help create the first subway? And what's the story with the tree dedication at the end of Europe's busiest shopping street? We send five local experts out to investigate some of London's surprising secrets, some buried underground, and others hiding (and driving) in plain sight. From the colossal Marble Arch to thumbnail-sized bone discs buried in the banks of the Thames, we examine remarkable objects great and small and unveil their hidden histories.