Economist Nouriel Roubini has made plenty of warnings about impending crashes or bubbles bursting, earning him the teasing nickname "Dr. Doom" for much of his career — including in 2006, when he predicted the impending collapse of the housing market. Roubini recently issued another warning: this time, about bitcoin. This MarketWatch story outlines why Roubini believes cryptocurrencies are "the mother of all scams and bubbles," and why he believes the current bubble could end badly.