As the sign explains (though the text is probably just a little too small in this photo) this is the old Washington family vault at Mount Vernon. George and Martha Washington along with assorted other family members were interred here until the new tomb (which George Washington had instructed be built) was completed in 1831.
It's certainly not a very imposing monument. But then I suspect Washington wasn't really anticipating the million or so people who visit Mount Vernon each year. In the absence of any really strong feature I tried to bring out the textures in the wooden door and the brickwork.