The original Cavendish Laboratory on Free School Lane is now part of the History and Philosophy of Science department, but the building's history is extraordinary. Opened in 1874, it was the site of more fundamental discoveries in physics than perhaps any other building in the world — the electron, the proton, the neutron, and the structure of DNA all identified within its walls.
A plaque on the wall marks the discoveries. The building itself is not open to the public, but you can stand outside on Free School Lane, walk past the Whipple Museum next door, and imagine Thomson, Rutherford, and Crick moving between these doors. The new Cavendish is now in West Cambridge; this old building remains a working university department.