The ruins at Baalbek, Lebanon have the largest (and heaviest) worked stone ever: this one above is a perfect rectangular block, estimated to weigh about 2,000 tons. So, how did ancient civilizations, without heavy machineries, make and transport these behemoths?
From the website:
The oldest part of the ruins at Baalbek fit absolutely no known culture, and were originally employed for some mysterious purpose. ...
The massive and elegant Roman stonework and columns pale by comparison to the megaliths they were built upon. The temple very visibly incorporates into its foundation, stones of some 1,500 tons. They are some 68 x 14 x 14 feet! They are the largest worked stones on earth! It is a mystery how such stones could have been moved into place, even according to our science and engineering knowledge of today. It is also a fact the Romans did not use this type of stonework.
Link - via Chapel Perilous