I've never been much interested in going to the Eiffel Tower, Big Ben, the Colosseum, or any of the standard tourist draws in Europe. When I travel, I like to track down some of the more specialized attractions - the ones that have always lured history's outsiders, the debauched aristocrats, wild monks and free-spirited noblewomen who populate the continent's secret past. Luckily, while researching "The Sinner's Grand Tour" (Broadway, May 2011), I discovered that Europe is still littered with underground historical sites. Seeking out these forgotten boudoirs, relics and dungeons provides travelers with an entirely fresh view of the most over-trodden destinations. Let your imagination run riot!