“A big humbug” – that’s how one critic described America’s first subway system. Other opponents were more hyperbolic. It would release dangerous underground air, some said. It would disturb the dead, others said. A religious leader in Boston declared it a project of Lucifer himself. Why were people so opposed to this new form of transportation? To understand it, we have to rewind centuries – to a time when people thought that Earth was hollow, and that hell was directly under their feet.