Deep below the streets of London, something is growing in tunnels that once kept people safe from World War II bombs. One hint: It’s leafy. Growing Underground is a company that makes “kilometer zero” eating possible in London, by growing salad in LED-lit, underground factories right beneath the customers’ feet. The Growing Underground farm sits 100 feet under Clapham in South London. It uses hydroponics and low-energy lighting to grow salad, including mizuna, watercress, Thai…