A secret subterranean heart, tinged with mystery and myth, beats beneath the streets in many of the world’s great cities. Tourists seek out the catacombs of Rome, the sewers of Paris and the subway tunnels of New York. Some people believe a den of interstellar aliens lurks beneath Denver International Airport. Now Seattle, at least for now, has joined that exclusive club. Something unknown, engineers say — and all the more intriguing to many residents for being unknown — has blocked the progress of the biggest-diameter tunnel-boring machine in use on the planet, a high-tech, largely automated wonder called Bertha....