The Mob Museum is opening a hidden Prohibition-era speakeasy. The Underground will be a permanent exhibition on the history of Prohibition that will double as a working speakeasy and distillery. It promises to be an only-in-Vegas spot for enjoying beer and cocktails while learning a thing or two about gangsters and flappers. The Underground is in the basement of the former Stewart Avenue post office and courthouse that houses the museum and will be open — and pouring cocktails and mocktails — during museum hours.