Book excerpt: Below Scotland's capital, hidden for almost two centuries, is a metropolis whose very existence was all but forgotten. For almost 250 years, Edinburgh was surrounded by a giant defensive wall. Unable to expand the city's boundaries, the burgeoning population built over every inch of square space. And when there was no more room, they began to dig down . . . Trapped in lives of poverty and crime, these subterranean dwellers existed in darkness and misery, ignored by the chroniclers of their time. It is only in the last few years that the shocking truth has begun to emerge about the sinister underground city.