If you discount CERN’s Large Hadron Collider (which is officially the Largest Machine in the World; see the Google Earth picture below, and read our article about it)
then the biggest MOVING machine build by humans would be the giant bucket wheel excavator “Bagger 288” built by Krupp in Germany in 1978.
This enormous mining machine (300 meters long, weight: 45,500 tons) is capable of moving on its treads to the various open-air mine locations.
The last such epic transition happened in 2001, when it crawled from one mine to the other – across 22 kilometers of fields, roads, railway crossings, villages and countryside… As it could not go around the obstacles, it headed in a straight line, so the workers prepared the way by piling up sand cushions on top of highways and train tracks, removing power-lines and seeding the fields with special grass to make its progress smoother.
More pics at the link