From a Harvard Law student:
Later in the day I took a walking tour of the Greenway (which is where the old highway was before the Big Dig submerged it).
There’s a little known secret that most people aren’t away of: there is an observation deck at the top of the old Customs House tower. The Customs House was originally built in 1847 as a three story greek style temple with a dome on top. In 1915, a tower was added over the dome, making it Boston’s first skyscraper. The observation deck is not advertised (to locals or tourists) and there are no signs in the building leading you to it. But you just walk into the lobby (the building is now a Marriott time-share) and take the elevator up to the 19th floor. Then you grab a second elevator to the 25th floor. It’s Boston’s only hi-rise outdoor observation deck and it offers the most amazing views.