The name alone was enough to make us want to see for ourselves. Fatbergs, the giant collections of grease, oil, and fat that clog the sewer system under London's streets, were getting out of hand, the result of decades of people washing things down their drains that they're not supposed to. It all ends up in the tunnels, along with the other usual things that run through raw sewage tunnels.

 

When I toured London's sewers earlier this year with National Geographic video producer Spencer Millsap, what we found was a potent cocktail of smells. Fungus covered every surface of the tunnels. The humidity was so great that our camera's lens instantly fogged up. Running around our feet was a shallow river colored an earthy, brown hue (exactly why you think).