Franklin Court was the site of the handsome brick home of Benjamin Franklin, who lived here while serving in the Continental Congress andthe Constitutional Convention. Franklin died here in 1790; the house was torn down about 20 years later. Today the site contains a steel "ghost structure" outlining the spot where Franklin's house stood and features an underground museum with a film and displays, an 18th century printing office, an architectural/archeological exhibit, an operating post office and a postal museum.
As you can see from the official website, the house itself was not underground, but there is today an underground museum. I was there a couple of years ago on 4th of July weekend (Gawd! It's hot in Philly in July!). One thing that amazed me was that Franklin Court has plaques on the ground that show the sites of various outbuildings (privies) and wells. At one point there is a well less than 10 feet from a privy! The well water must've had an interesting flavor. When Ben lived there (for that matter, everywhere he lived), he could be seen taking an "air bath"; that is, he would sit naked in a well ventilated room allowing a breeze to "wash off the stink". Old Ben was an interesting character.
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I'd rather be historically accurate than politically correct.